libgui: Enable -Weverything and -Werror

Enables -Weverything and -Werror, with just a few exceptions for
warnings we can't (or shouldn't need to) work around.

This is a squashed commit based on an initial change with a couple of
fixes to avoid breaking certain targets. The source commits are:
  d723bd7669b4fc88dc282d8bf8ba5ecb2849d22f
  00d504c06ea034befe143e6b8cb34d004670ed02
  429ba89cd293633d2f882165066b422c96597df2

Change-Id: I034abec27bf4020d84af60d7acc1939c59986dd6
diff --git a/libs/gui/SyncFeatures.cpp b/libs/gui/SyncFeatures.cpp
index e5804a7..187b211 100644
--- a/libs/gui/SyncFeatures.cpp
+++ b/libs/gui/SyncFeatures.cpp
@@ -16,7 +16,6 @@
 
 #define LOG_TAG "GLConsumer"
 
-#define GL_GLEXT_PROTOTYPES
 #define EGL_EGLEXT_PROTOTYPES
 
 #include <EGL/egl.h>
@@ -78,10 +77,11 @@
     // on some devices it's better to not use EGL_KHR_fence_sync
     // even if they have it
     return false;
-#endif
+#else
     // currently we shall only attempt to use EGL_KHR_fence_sync if
     // USE_FENCE_SYNC is set in our makefile
     return !mHasNativeFenceSync && mHasFenceSync;
+#endif
 }
 bool SyncFeatures::useWaitSync() const {
     return (useNativeFenceSync() || useFenceSync()) && mHasWaitSync;