makefile.unix: add rudimentary avx2 support
makefile.unix: add rudimentary avx2 support

$ make -f makefile.unix HAVE_AVX2=1

will define -mavx2 for src/dsp/*_dsp.c

Change-Id: Id9651bda54da057cb051dc70f7dcd008a3f803f4

Acidburn0zzz/webm.libwebp,keane83/BLE_IM,reimaginemedia/webm.libwebp,PKRoma/libwebp,Maria1099/webm.libwebp,keane83/BLE_IM,redengineer/libwebp,ericmckean/webm.libwebp,keane83/BLE_IM,imazen/libwebp,jiehu5114/libwebp,kim42083/webm.libwebp,webmproject/libwebp,Aexyn/libwebp,ericmckean/webm.libwebp,PKRoma/libwebp,gshORTON/webm.libwebp,iniwf/webm.libwebp,matanbs/webm.libwebp,PKRoma/libwebp,jiehu5114/libwebp,Maria1099/webm.libwebp,PKRoma/libwebp,Acidburn0zzz/webm.libwebp,comoyo/gonzo-libwebp,jacklicn/libwebp,kalli123/webm.libwebp,ttyangf/libwebp,Suvarna1488/webm.libwebp,imazen/libwebp,zofuthan/libwebp,abwiz0086/webm.libwebp,PKRoma/libwebp,ericmckean/webm.libwebp,ya7lelkom/libwebp,Maria1099/webm.libwebp,kalli123/webm.libwebp,keane83/BLE_IM,appunite/libwebp,comoyo/gonzo-libwebp,matanbs/webm.libwebp,kleopatra999/webm.libwebp,Acidburn0zzz/webm.libwebp,ttyangf/libwebp,appunite/libwebp,ttyangf/libwebp,matanbs/webm.libwebp,iniwf/webm.libwebp,abwiz0086/webm.libwebp,gshORTON/webm.libwebp,imazen/libwebp,kleopatra999/webm.libwebp,Suvarna1488/webm.libwebp,matanbs/webm.libwebp,kim42083/webm.libwebp,redengineer/libwebp,ttyangf/libwebp,comoyo/gonzo-libwebp,iniwf/webm.libwebp,rxl194/libwebp,PKRoma/libwebp,abwiz0086/webm.libwebp,kim42083/webm.libwebp,reimaginemedia/webm.libwebp,iniwf/webm.libwebp,Maria1099/webm.libwebp,zofuthan/libwebp,webmproject/libwebp,altogother/webm.libwebp,yohunl/libwebp,Maria1099/webm.libwebp,rxl194/libwebp,zofuthan/libwebp,jacklicn/libwebp,redengineer/libwebp,Aexyn/libwebp,Aexyn/libwebp,Suvarna1488/webm.libwebp,yohunl/libwebp,Aexyn/libwebp,comoyo/gonzo-libwebp,jacklicn/libwebp,kalli123/webm.libwebp,Suvarna1488/webm.libwebp,reimaginemedia/webm.libwebp,zofuthan/libwebp,appunite/libwebp,Suvarna1488/webm.libwebp,Acidburn0zzz/webm.libwebp,appunite/libwebp,Aexyn/libwebp,imazen/libwebp,comoyo/gonzo-libwebp,keane83/BLE_IM,yohunl/libwebp,jiehu5114/libwebp,comoyo/gonzo-libwebp,webmproject/libwebp,yohunl/libwebp,Acidburn0zzz/webm.libwebp,kleopatra999/webm.libwebp,ya7lelkom/libwebp,jiehu5114/libwebp,kleopatra999/webm.libwebp,yohunl/libwebp,ya7lelkom/libwebp,redengineer/libwebp,appunite/libwebp,gshORTON/webm.libwebp,iniwf/webm.libwebp,redengineer/libwebp,ericmckean/webm.libwebp,kalli123/webm.libwebp,appunite/libwebp,jacklicn/libwebp,altogother/webm.libwebp,ya7lelkom/libwebp,reimaginemedia/webm.libwebp,rxl194/libwebp,abwiz0086/webm.libwebp,ya7lelkom/libwebp,matanbs/webm.libwebp,altogother/webm.libwebp,rxl194/libwebp,kim42083/webm.libwebp,imazen/libwebp,kim42083/webm.libwebp,reimaginemedia/webm.libwebp,kleopatra999/webm.libwebp,zofuthan/libwebp,comoyo/gonzo-libwebp,rxl194/libwebp,jacklicn/libwebp,gshORTON/webm.libwebp,gshORTON/webm.libwebp,altogother/webm.libwebp,kalli123/webm.libwebp,abwiz0086/webm.libwebp,jiehu5114/libwebp,webmproject/libwebp,ttyangf/libwebp,altogother/webm.libwebp,webmproject/libwebp,ericmckean/webm.libwebp