configure.ac: Get complete LLVM version from header

configure.ac: Get complete LLVM version from header

Major and minor version are included in the header file since LLVM
version 3.1.0. Since the minimal required version is 3.3.0 we can
remove the workaround if no values for major/minor were found in the
header.

Since LLVM 3.6.0 the patch version is inside the header file of LLVM.
Only radeon drivers need the patch version and they depend on
LLVM >= 3.6.0, so this is safe too.

Signed-off-by: Tobias Droste <tdroste@gmx.de>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>