Add "WITH_SYNTAX_CHECK=1 make ..."

The new option WITH_SYNTAX_CHECK=1 instructs build system to invoke
"clang -fsyntax-only" to utilize clang's better diagnostics before calling
LOCAL_CC/LOCAL_CXX for code generation.  The compilation time is slightly
longer, and the generated object file should be the same as w/o WITH_SYNTAX_CHECK

Project use lots of GCC extensions (eg. nested function) not supported
by clang may opt out by adding LOCAL_NO_SYNTAX_CHECK:=true

Change-Id: I5689586788ef049bd967364f71f31f1e359bd121
diff --git a/core/clear_vars.mk b/core/clear_vars.mk
index 7ad58b2..140cfa2 100644
--- a/core/clear_vars.mk
+++ b/core/clear_vars.mk
@@ -137,6 +137,7 @@
 LOCAL_PREBUILT_MODULE_FILE:=
 LOCAL_POST_INSTALL_CMD:=
 LOCAL_DIST_BUNDLED_BINARIES:=
+LOCAL_NO_SYNTAX_CHECK:=
 
 # Trim MAKEFILE_LIST so that $(call my-dir) doesn't need to
 # iterate over thousands of entries every time.