makefile.unix: rule maintenance
makefile.unix: rule maintenance

- use built-in variables where appropriate
- fix some target dependencies

Change-Id: I33584d0e7d9c5341841da89b6aa8f02e1c7c6aea

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