makefile.unix: provide examples/webpmux target
makefile.unix: provide examples/webpmux target

this target is experimental so must be explicitly set

missing since:
e41a759 build: remove libwebpmux from default targets/config

Change-Id: I128d9e6a06ba3e7102ae3175dc3db52c6f4b1439

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