configure.ac: Use POSIX compatible regex for word boundary.
configure.ac: Use POSIX compatible regex for word boundary.

Fixes build error on Mac OS X.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=100236
Suggested-by: Jan Beich <93439dd42ccb629b9c9ae9f8c3f0643f544d9574@freebsd.org>
Suggested-by: Michel Dänzer <2c7f9fd20fbeb41ce8894ec4653d66fa7f3b6e1a@daenzer.net>
Signed-off-by: Vinson Lee <25a259f709b0f1b620605d2518271a846d8826e5@freedesktop.org>
Reviewed-by: Eric Engestrom <96f164ad4d9b2b0dacf8ebee2bb1eeb3aa69adf1@engestrom.ch>
Acked-by: Emil Velikov <f20878473f414da183da19ee25c8722cc1d711e4@collabora.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler