configure.sh fix for visual studio

-For targets with external build systems like visual
studio CC is not set so check_add_cflags will fail.
Only call this function if extra_cflags is set.

Change-Id: I3531bad69e9b6a59c5be1b0e8b6053ccccbc332c

build/make/configure.sh

@@ -979,8 +979,10 @@ EOF
     fi
 
     # append any user defined extra cflags
-    check_add_cflags ${extra_cflags} || \
-    die "Requested extra CFLAGS '${extra_cflags}' not supported by compiler"
+    if [ -n "${extra_cflags}" ] ; then
+        check_add_cflags ${extra_cflags} || \
+        die "Requested extra CFLAGS '${extra_cflags}' not supported by compiler"
+    fi
 }
 
 process_toolchain() {