Fix compiler detection and let Qt decide if examples should be built

This fixes the build on OS X if the path to clang is not cached,
and only builds examples if Qt was configured to do so.
This has already been fixed in dev.

Change-Id: Ia2d17f38f30e832425f6e1cd014b46a6c6ae78ec
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@theqtcompany.com>

qtwebengine.pro

 load(qt_build_config)
-
-# As long as we are a module separate from the rest of Qt, we want to unconditionally build examples.
-# Once part of Qt 5, this should be removed and we should respect the Qt wide configuration.
-QTWEBENGINE_BUILD_PARTS = $$QT_BUILD_PARTS
-QTWEBENGINE_BUILD_PARTS *= examples
-
 load(qt_parts)

tools/qmake/mkspecs/features/default_pre.prf

@@ -11,6 +11,8 @@ MODULE_BASE_DIR = $$QTWEBENGINE_ROOT
 
 EOL = $$escape_expand(\\n)
 
+# Call the original default_pre.
+load(default_pre)
 load(functions)
 
 # Check platform support and run config tests early enough to bail
@@ -18,6 +20,3 @@ equals(_PRO_FILE_, "$$QTWEBENGINE_ROOT/qtwebengine.pro"): isPlatformSupported()
    load(configure)
    runConfigure()
 }
-
-# Call the original default_pre.
-load(default_pre)