automake: warnings are treated as errors
Makefile.am:4: warning: source file 'zlib/adler32.c' is in a subdirectory,
Makefile.am:4: but option 'subdir-objects' is disabled
automake: warning: possible forward-incompatibility.
automake: At least a source file is in a subdirectory, but the 'subdir-objects'
automake: automake option hasn't been enabled.  For now, the corresponding output
automake: object file(s) will be placed in the top-level directory.  However,
automake: this behaviour will change in future Automake versions: they will
automake: unconditionally cause object files to be placed in the same subdirectory
automake: of the corresponding sources.
automake: You are advised to start using 'subdir-objects' option throughout your
automake: project, to avoid future incompatibilities.
Makefile.am:4: warning: source file 'zlib/compress.c' is in a subdirectory,
Makefile.am:4: but option 'subdir-objects' is disabled
Makefile.am:4: warning: source file 'zlib/crc32.c' is in a subdirectory,
Makefile.am:4: but option 'subdir-objects' is disabled
Makefile.am:4: warning: source file 'zlib/deflate.c' is in a subdirectory,
Makefile.am:4: but option 'subdir-objects' is disabled
Makefile.am:4: warning: source file 'zlib/inffast.c' is in a subdirectory,
Makefile.am:4: but option 'subdir-objects' is disabled
Makefile.am:4: warning: source file 'zlib/inflate.c' is in a subdirectory,
Makefile.am:4: but option 'subdir-objects' is disabled
Makefile.am:4: warning: source file 'zlib/inftrees.c' is in a subdirectory,
Makefile.am:4: but option 'subdir-objects' is disabled
Makefile.am:4: warning: source file 'zlib/trees.c' is in a subdirectory,
Makefile.am:4: but option 'subdir-objects' is disabled
Makefile.am:4: warning: source file 'zlib/uncompr.c' is in a subdirectory,
Makefile.am:4: but option 'subdir-objects' is disabled
Makefile.am:4: warning: source file 'zlib/zutil.c' is in a subdirectory,
Makefile.am:4: but option 'subdir-objects' is disabled