makefile.unix: allow *_LIBS to be overridden w/EXTRA_LIBS
makefile.unix: allow *_LIBS to be overridden w/EXTRA_LIBS

previously setting EXTRA_LIBS on the command line would skip any
per-target updates. this allows both EXTRA_LIBS and [CD]WEBP_LIBS,
GL_LIBS to be set together avoiding the need to add unnecessary
dependencies to EXTRA_LIBS which affects all targets.

Change-Id: I63bbc14f0ac0cd04aa50c5d5047060ac57d0c9dc

PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp