configure.ac: Only define HAVE_LLVM if LLVM is used
configure.ac: Only define HAVE_LLVM if LLVM is used

Make sure that HAVE_LLVM compiler define is only set if LLVM is
actually used.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=99010
Cc: "17.0" <59f39c0db42d4479a46b02d4d2bc11120e37bb44@lists.freedesktop.org>
Signed-off-by: Tobias Droste <6f0b7afb8e169bccf102c27c4328261728913687@gmx.de>
v2 [Emil] fold within the existing conditional
Reviewed-by: Emil Velikov <f20878473f414da183da19ee25c8722cc1d711e4@collabora.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler