This is only needed when building with WEBP support, which uses atomics,
therefore the linker needs the --shared-memory flag. The flag cannot be
added globally because not all executables link against libwebp.
libtiff mailing list on Mon, 22 Jun 2015 21:21:01 +0100. Several
corrections to ensure that the autotools build still works were
added by me. I have not yet tested the build using 'cmake' or
MSVC with 'nmake'.