this is conditional on whether the linker supports version scripts, for
which we use the gl_LD_VERSION_SCRIPT macro from the gnulib project.
on platforms without version scripts, we fall back to libtool's
-export-symbols-regex.
Still need to add the comments in the source code. Currently I've
seeded it with the libprotobuf-c files. I've configured it
to make man pages and html pages. Might not be ideal, but makes it easy
for me to check things (html is nicer, but man pages are handier for
remote servers).
Makefile.am: add valgrind to the AM_TESTS_ENVIRONMENT
configure.ac: enable valgrind testing option for ./configure
m4/valgrind-tests.m4: enable tracing children for libtool wrapper
script compatibility, but ignore standard binaries in /usr or /bin
