reintroduce BUILD_LDFLAGS for the cross-compile case

reintroduce BUILD_LDFLAGS for the cross-compile case

This allows providing separate LDFLAGS for build and host environments, which
are not necessary the same for cross-compile case. In particular, it allows
building host programs statically to not depend on presence of libraries at
run-time (e.g. MinGW DLLs on Windows) while continue to use default dynamic
linking at build time.

Also fix obsolete comments in confgure.ac - we do use different environment
for build and host now.