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).
git: Add gphoto m4 macros as submodule */m4m

Add git submodules for the gphoto m4 macros as a replacement for
the SVN svn:externals mechanism. We need the macros twice, namely
in m4m/ and libgphoto2_port/m4m/.

Note that the libgphoto2_port/m4m/ location is different from the
original location libgphoto2_port/m4/ in order to make the layout
more uniform.
git: Adapt INSTALL instructions for git submodule