Set the correct CMake option CMAKE_CROSSCOMPILING in build-llvm.sh

The previous option doesn't seem to have had any effect, and cmake
correctly has warned about an unused option.

Also set the value of the option to TRUE, for slightly more consistency
with where this option is set in build-llvm.sh.

build-llvm.sh

@@ -109,7 +109,7 @@ if [ -n "$HOST" ]; then
     fi
 
     CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_SYSTEM_NAME=Windows"
-    CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_CROSSCOMPILE=1"
+    CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_CROSSCOMPILING=TRUE"
     CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_C_COMPILER=$HOST-gcc"
     CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_CXX_COMPILER=$HOST-g++"
     CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_RC_COMPILER=$HOST-windres"