git: Add gphoto m4 macros as submodule m4m

Add git submodule for the gphoto m4 macros into m4m/ as a replacement
for the SVN svn:externals mechanism.
git: ignore files in srcdir generated during build

This ignores only files generated in $(srcdir).

This does *not* include the majority of the files generated during
builds which are placed into $(builddir).