Add plugin constructor registration for all libraries that provide plugins

Unfortunately, we can't just add the generated C file to the sources in
Makefile.am as the linker would remove that object file when it notices
that no symbol in it is ever referenced.  So we include it in the file
that contains the library initialization, which will definitely be
referenced by the executable.

This allows building an almost stand-alone static version of e.g. charon
when building with `--enable-monolithic --enable-static --disable-shared`
(without `--disable-shared` libtool will only build a version that links
the libraries dynamically).  External libraries (e.g. gmp or openssl) are
not linked statically this way, though.

.gitignore

@@ -27,6 +27,7 @@ libtool
 y.tab.[ch]
 lex.yy.c
 *keywords.c
+plugin_constructors.c
 Doxyfile
 apidoc/
 *~