1. fix portlint complaint about incorrect whitespace in CONFIGURE_ARGS
2. instead of unconditionally setting LIBS in CONFIGURE_ENV use automake logc
to correctly set library dependencies for the case when external libmp4v2
Fix #2 also fixes build with non-base gcc: configure stage would pass LIBS
to all invocations of gcc and linking would fail because libmp4v2 is a C++
library, but non-base gcc (as opposed to g++) doesn't know where to look
for the correct libstdc++.
Submitted by: Andriy Gapon <firstname.lastname@example.org>