Makefile-defs.linux: do not override user CFLAGS

fixes #87
Makefile-defs.linux: do not call pkg-config on every LIBS eval
Merge pull request #90 from rofl0r/respect_user_cflags

Makefile-defs.linux: do not override user CFLAGS
Makefile: Display executed commands when using "make V=1"
Merge branch 'release-3.3.0' of github.com:scottchiefbaker/ags into release-3.3.0
Update the --help output