CppTools: Re-introduce gcc 4.9 __has_include fix

Now the ClangCodeModel makes also use of this.

Change-Id: I3d547dffe48e35d3b3d03063bc1a640283d670c1
Reviewed-by: Christian Kandeler <christian.kandeler@theqtcompany.com>

src/plugins/cpptools/cppprojects.cpp

@@ -705,6 +705,16 @@ bool CompilerOptionsBuilder::excludeDefineLine(const QByteArray &defineLine) con
     if (defineLine.startsWith("#define __cplusplus"))
         return true;
 
+    // gcc 4.9 has:
+    //    #define __has_include(STR) __has_include__(STR)
+    //    #define __has_include_next(STR) __has_include_next__(STR)
+    // The right-hand sides are gcc built-ins that clang does not understand, and they'd
+    // override clang's own (non-macro, it seems) definitions of the symbols on the left-hand
+    // side.
+    const bool isGccToolchain = m_projectPart->toolchainType == QLatin1String("gcc");
+    if (isGccToolchain && defineLine.contains("has_include"))
+        return true;
+
     return false;
 }