Driver: Honor %INCLUDE% when built with MinGW

Users are expected to pass system includes through the INCLUDE
environment variable on Windows.  There's no reason to change behavior
based on the toolchain used to build Clang.

I didn't change the registry searching code because I'm not sure it
builds with mingw and I'm not set up to test it.

llvm-svn: 206934

clang/lib/Driver/WindowsToolChain.cpp

@@ -293,7 +293,6 @@ void Windows::AddClangSystemIncludeArgs(const ArgList &DriverArgs,
   if (DriverArgs.hasArg(options::OPT_nostdlibinc))
     return;
 
-#ifdef _MSC_VER
   // Honor %INCLUDE%. It should know essential search paths with vcvarsall.bat.
   if (const char *cl_include_dir = getenv("INCLUDE")) {
     SmallVector<StringRef, 8> Dirs;
@@ -305,6 +304,7 @@ void Windows::AddClangSystemIncludeArgs(const ArgList &DriverArgs,
       return;
   }
 
+#ifdef _MSC_VER
   std::string VSDir;
   std::string WindowsSDKDir;
 

clang/test/Driver/windows-include-env.c

@@ -0,0 +1,4 @@
+// RUN: env INCLUDE=my_system_headers %clang -c %s -### 2>&1 | FileCheck %s
+// RUN: env INCLUDE=my_system_headers %clang_cl -c %s -### 2>&1 | FileCheck %s
+// CHECK: "-cc1"
+// CHECK: "-internal-isystem" "my_system_headers"