Make sure $CFG_LLVM_CONFIG is set to $CFG_LLVM_ROOT/bin/llvm-config

On my machine I have two LLVM builds, one of regular HEAD and one
for Rust in ~/rust-llvm - by default CFG_LLVM_CONFIG is set to
/usr/local/bin/llvm-config which is wrong, because the probe for it
initially happens earlier in configure and succeeds (so putvar is called.)

This causes it to be emitted twice into the Makefile but the second
instance wins.

Author: Austin Seipp

configure

@@ -371,6 +371,7 @@ putvar CFG_C_COMPILER
 putvar CFG_PREFIX
 
 putvar CFG_LLVM_ROOT
+putvar CFG_LLVM_CONFIG
 putvar CFG_LLVM_INCDIR
 putvar CFG_LLVM_BINDIR
 putvar CFG_LLVM_LIBDIR