makefile: fix .so build rule
makefile: fix .so build rule

Use $^ vs $? to capture all prerequisites, rather than only newer ones.

Change-Id: I464ffb2913cf60f5ca5e07518ad4bf7da10baf87

GrokImageCompression/aom,cinema6/libvpx,lyx2014/libvpx_c,vasilvv/esvp8,altogother/webm.libvpx,smarter/aom,jmvalin/aom,mwgoldsmith/libvpx,WebRTC-Labs/libvpx,lyx2014/libvpx_c,vasilvv/esvp8,altogother/webm.libvpx,Suvarna1488/webm.libvpx,shareefalis/libvpx,jdm/libvpx,liqianggao/libvpx,charup/https---github.com-webmproject-libvpx-,vasilvv/esvp8,zofuthan/libvpx,kalli123/webm.libvpx,goodleixiao/vpx,matanbs/webm.libvpx,mwgoldsmith/libvpx,webmproject/libvpx,jmvalin/aom,mwgoldsmith/vpx,cinema6/libvpx,ittiamvpx/libvpx,thdav/aom,ittiamvpx/libvpx-1,turbulenz/libvpx,Topopiccione/libvpx,reimaginemedia/webm.libvpx,Acidburn0zzz/webm.libvpx,shareefalis/libvpx,stewnorriss/libvpx,liqianggao/libvpx,shareefalis/libvpx,cinema6/libvpx,iniwf/webm.libvpx,shareefalis/libvpx,gshORTON/webm.libvpx,Topopiccione/libvpx,reimaginemedia/webm.libvpx,zofuthan/libvpx,gshORTON/webm.libvpx,running770/libvpx,ittiamvpx/libvpx-1,shacklettbp/aom,zofuthan/libvpx,Topopiccione/libvpx,thdav/aom,pcwalton/libvpx,pcwalton/libvpx,ittiamvpx/libvpx-1,zofuthan/libvpx,kleopatra999/webm.libvpx,stewnorriss/libvpx,jdm/libvpx,kalli123/webm.libvpx,mbebenita/aom,reimaginemedia/webm.libvpx,cinema6/libvpx,cinema6/libvpx,abwiz0086/webm.libvpx,kleopatra999/webm.libvpx,mwgoldsmith/vpx,n4t/libvpx,iniwf/webm.libvpx,mwgoldsmith/vpx,Distrotech/libvpx,matanbs/vp982,hsueceumd/test_hui,stewnorriss/libvpx,n4t/libvpx,hsueceumd/test_hui,shacklettbp/aom,turbulenz/libvpx,ittiamvpx/libvpx,smarter/aom,charup/https---github.com-webmproject-libvpx-,WebRTC-Labs/libvpx,thdav/aom,matanbs/webm.libvpx,shyamalschandra/libvpx,reimaginemedia/webm.libvpx,vasilvv/esvp8,webmproject/libvpx,shyamalschandra/libvpx,ShiftMediaProject/libvpx,Acidburn0zzz/webm.libvpx,jdm/libvpx,felipebetancur/libvpx,Distrotech/libvpx,jacklicn/webm.libvpx,kim42083/webm.libvpx,stewnorriss/libvpx,running770/libvpx,VTCSecureLLC/libvpx,Maria1099/webm.libvpx,Suvarna1488/webm.libvpx,felipebetancur/libvpx,matanbs/vp982,WebRTC-Labs/libvpx,shacklettbp/aom,altogother/webm.libvpx,altogother/webm.libvpx,openpeer/libvpx_new,Topopiccione/libvpx,webmproject/libvpx,jdm/libvpx,iniwf/webm.libvpx,goodleixiao/vpx,n4t/libvpx,abwiz0086/webm.libvpx,sanyaade-teachings/libvpx,luctrudeau/aom,turbulenz/libvpx,sanyaade-teachings/libvpx,hsueceumd/test_hui,Distrotech/libvpx,luctrudeau/aom,jacklicn/webm.libvpx,charup/https---github.com-webmproject-libvpx-,kalli123/webm.libvpx,Suvarna1488/webm.libvpx,matanbs/vp982,abwiz0086/webm.libvpx,jacklicn/webm.libvpx,Distrotech/libvpx,matanbs/vp982,turbulenz/libvpx,Maria1099/webm.libvpx,shareefalis/libvpx,vasilvv/esvp8,Maria1099/webm.libvpx,matanbs/webm.libvpx,GrokImageCompression/aom,pcwalton/libvpx,mbebenita/aom,pcwalton/libvpx,lyx2014/libvpx_c,shacklettbp/aom,openpeer/libvpx_new,kleopatra999/webm.libvpx,openpeer/libvpx_new,jdm/libvpx,Acidburn0zzz/webm.libvpx,iniwf/webm.libvpx,mbebenita/aom,jacklicn/webm.libvpx,turbulenz/libvpx,Acidburn0zzz/webm.libvpx,jmvalin/aom,kalli123/webm.libvpx,sanyaade-teachings/libvpx,shacklettbp/aom,kim42083/webm.libvpx,liqianggao/libvpx,jmvalin/aom,liqianggao/libvpx,Laknot/libvpx,Laknot/libvpx,altogother/webm.libvpx,Suvarna1488/webm.libvpx,abwiz0086/webm.libvpx,lyx2014/libvpx_c,Laknot/libvpx,pcwalton/libvpx,cinema6/libvpx,charup/https---github.com-webmproject-libvpx-,Topopiccione/libvpx,smarter/aom,VTCSecureLLC/libvpx,goodleixiao/vpx,mwgoldsmith/vpx,ShiftMediaProject/libvpx,lyx2014/libvpx_c,gshORTON/webm.libvpx,luctrudeau/aom,mbebenita/aom,Maria1099/webm.libvpx,shyamalschandra/libvpx,gshORTON/webm.libvpx,ittiamvpx/libvpx,GrokImageCompression/aom,hsueceumd/test_hui,Distrotech/libvpx,iniwf/webm.libvpx,smarter/aom,cinema6/libvpx,kim42083/webm.libvpx,ittiamvpx/libvpx,vasilvv/esvp8,smarter/aom,zofuthan/libvpx,Acidburn0zzz/webm.libvpx,kim42083/webm.libvpx,ittiamvpx/libvpx,thdav/aom,VTCSecureLLC/libvpx,felipebetancur/libvpx,Laknot/libvpx,matanbs/vp982,mwgoldsmith/libvpx,openpeer/libvpx_new,mbebenita/aom,kalli123/webm.libvpx,running770/libvpx,reimaginemedia/webm.libvpx,kleopatra999/webm.libvpx,mwgoldsmith/libvpx,ShiftMediaProject/libvpx,vasilvv/esvp8,matanbs/webm.libvpx,GrokImageCompression/aom,Laknot/libvpx,shyamalschandra/libvpx,running770/libvpx,mwgoldsmith/vpx,smarter/aom,luctrudeau/aom,abwiz0086/webm.libvpx,webmproject/libvpx,GrokImageCompression/aom,reimaginemedia/webm.libvpx,kalli123/webm.libvpx,thdav/aom,jmvalin/aom,WebRTC-Labs/libvpx,openpeer/libvpx_new,n4t/libvpx,liqianggao/libvpx,iniwf/webm.libvpx,mbebenita/aom,webmproject/libvpx,ittiamvpx/libvpx-1,turbulenz/libvpx,jacklicn/webm.libvpx,VTCSecureLLC/libvpx,hsueceumd/test_hui,luctrudeau/aom,running770/libvpx,jacklicn/webm.libvpx,altogother/webm.libvpx,VTCSecureLLC/libvpx,abwiz0086/webm.libvpx,Acidburn0zzz/webm.libvpx,gshORTON/webm.libvpx,sanyaade-teachings/libvpx,charup/https---github.com-webmproject-libvpx-,ShiftMediaProject/libvpx,mwgoldsmith/libvpx,hsueceumd/test_hui,shareefalis/libvpx,shacklettbp/aom,mwgoldsmith/libvpx,WebRTC-Labs/libvpx,matanbs/vp982,Maria1099/webm.libvpx,Distrotech/libvpx,goodleixiao/vpx,thdav/aom,zofuthan/libvpx,mbebenita/aom,matanbs/webm.libvpx,stewnorriss/libvpx,ittiamvpx/libvpx-1,kleopatra999/webm.libvpx,shyamalschandra/libvpx,kleopatra999/webm.libvpx,ittiamvpx/libvpx-1,turbulenz/libvpx,felipebetancur/libvpx,goodleixiao/vpx,jmvalin/aom,shyamalschandra/libvpx,webmproject/libvpx,stewnorriss/libvpx,sanyaade-teachings/libvpx,kim42083/webm.libvpx,mwgoldsmith/vpx,mbebenita/aom,felipebetancur/libvpx,liqianggao/libvpx,Suvarna1488/webm.libvpx,ShiftMediaProject/libvpx,VTCSecureLLC/libvpx,felipebetancur/libvpx,goodleixiao/vpx,lyx2014/libvpx_c,running770/libvpx,matanbs/webm.libvpx,matanbs/vp982,n4t/libvpx,Laknot/libvpx,charup/https---github.com-webmproject-libvpx-,gshORTON/webm.libvpx,mbebenita/aom,ittiamvpx/libvpx,Suvarna1488/webm.libvpx,Topopiccione/libvpx,luctrudeau/aom,turbulenz/libvpx,jdm/libvpx,openpeer/libvpx_new,GrokImageCompression/aom,Maria1099/webm.libvpx,pcwalton/libvpx,kim42083/webm.libvpx,turbulenz/libvpx