configure tweaks
These patches with some autotools issues:
First patch:
- use AM_SILENT_RULES if available
- add configure switch for building the API docs
- fix mistake in valgrind-testing switch, where $ac_enable_valgrind was always set to "yes" if --enable-valgrind-testing OR --disable-valgrind-testing was passed
- use AS_HELP_STRING for ./configure --help niceness
Second patch:
- don't set docdir in doc/{,spec/}Makefile.am so that docdir can actually be set with --docdir=foo
Best regards Marvin Schmidt