version the libprotobuf-c symbols on compatible linkers (Issue #116)

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.
configure.ac: make pkg-config optional when building with --disable-protoc

if pkg-config is installed, the libprotobuf-c .pc file will be
installed; if pkg-config is not installed, the .pc file won't be
installed.

this behavior only applies when we're building with ./configure
--disable-protoc, since pkg-config is required in order to detect the
protobuf dependency.