configure.ac: Don't search llvm-config if it's known
configure.ac: Don't search llvm-config if it's known

This way LLVM_CONFIG can bet set from an env variable if it's outside
the $llvm_prefix.

This is not a must, but it helps testing.

Signed-off-by: Tobias Droste <6f0b7afb8e169bccf102c27c4328261728913687@gmx.de>
Reviewed-by: Emil Velikov <f20878473f414da183da19ee25c8722cc1d711e4@collabora.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler