Added -IM4 to autoreconf for libsndfile.
libpng: update
flex: disable shared for $(BUILD) target
harfbuzz: update
new package: gendef

Like widl, the code ships with mingw-w64. This tool is useful
for generating .def files from dlls, especially x86_64 dlls.
Merge pull request #556 from eivindkv/master

Added -IM4 to autoreconf for libsndfile.
mxe-conf.cmake: Add Boost_THREADAPI

Closes #560.