configure.ac: Avoid running llvm-config when it hadn't been checked for.

configure.ac: Avoid running llvm-config when it hadn't been checked for.

If --disable-gallium is passed, llvm-config isn't checked for, so mark
it explicitly as absent, through LLVM_CONFIG=no.

Passing --disable-gallium would result in:
| ../configure: line 9739: --version: command not found
| ../configure: line 9740: --cppflags: command not found
| ../configure: line 9741: --libs: command not found
| ../configure: line 9743: --ldflags: command not found

With this commit, one gets that instead:
| configure: error: LLVM is required to build Gallium R300 on x86 and x86_64

Signed-off-by: Cyril Brulebois <kibi@debian.org>