configure.ac: Check for all known Python3 pkg-config names.

configure.ac: Check for all known Python3 pkg-config names.

Even when using pkg-config not all distro/platform specific issues are
transparent, unfortunately. On some systems the pkg-config file is
named "python3.pc" on others it's versioned, e.g. "python-3.2.pc".

See also: http://sigrok.org/wiki/Libsigrokdecode/Python

This should fix the build on e.g. FreeBSD, NetBSD, Mac OS X (some
versions), and possibly various Linux distros, e.g. Gentoo.

Also, drop manual AC_MSG_ERROR() and let PKG_CHECK_MODULES() output a
more verbose and standardized error message if Python 3 is not found.