build-llvm: Allow prepopulating CMAKEFLAGS

This allows passing options without proper command line flags.

build-llvm.sh

@@ -144,6 +144,8 @@ else
     BUILDCMD=make
 fi
 
+CMAKEFLAGS="$LLVM_CMAKEFLAGS"
+
 if [ -n "$HOST" ]; then
     CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_SYSTEM_NAME=Windows"
     CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_CROSSCOMPILING=TRUE"