The HOSTCC value defaults to the value of CC if not cross-compiling.
If cross-compiling, HOSTCC defaults to "gcc".

HOSTCFLAGS, HOSTLDFLAGS, and HOSTLIBS are also given sensible default
values.

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@15912 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b