Partially revert "Replace static with thread_local when used for caching"

As noticed by Stephan, clang on Mac is nowhere near to support thread_local for
some reason:
http://stackoverflow.com/questions/28094794/why-does-apple-clang-disallow-c11-thread-local-when-official-clang-supports

It does not look realistic to me to provide a configure-based alternative. The
solution is to not use thread_local until it is reasonably supported on
Mac. According to sources, this means requiring Xcode >= 8.

This reverts commit 82d4f1a446 partially.

src/frontends/qt4/GuiFontLoader.cpp

@@ -373,13 +373,9 @@ GuiFontInfo::GuiFontInfo(FontInfo const & f)
 
 bool FontLoader::available(FontInfo const & f)
 {
-#if defined(__GNUC__) && (__GNUC__ == 4) && (__GNUC_MINOR__ == 6)
-	static vector<bool> cache_set(NUM_FAMILIES, false);
-	static vector<bool> cache(NUM_FAMILIES, false);
-#else
-	thread_local vector<bool> cache_set(NUM_FAMILIES, false);
-	thread_local vector<bool> cache(NUM_FAMILIES, false);
-#endif
+	// FIXME THREAD
+	static vector<int> cache_set(NUM_FAMILIES, false);
+	static vector<int> cache(NUM_FAMILIES, false);
 
 	FontFamily family = f.family();
 #ifdef Q_OS_MAC

src/frontends/qt4/GuiPainter.cpp

@@ -208,11 +208,8 @@ void GuiPainter::lines(int const * xp, int const * yp, int np,
 		return;
 
 	// double the size if needed
-#if defined(__GNUC__) && (__GNUC__ == 4) && (__GNUC_MINOR__ == 6)
+	// FIXME THREAD
 	static QVector<QPoint> points(32);
-#else
-	thread_local QVector<QPoint> points(32);
-#endif
 	if (np > points.size())
 		points.resize(2 * np);
 

src/frontends/qt4/GuiSymbols.cpp

@@ -152,11 +152,8 @@ const int no_blocks = sizeof(unicode_blocks) / sizeof(UnicodeBlocks);
 QString getBlock(char_type c)
 {
 	// store an educated guess for the next search
-#if defined(__GNUC__) && (__GNUC__ == 4) && (__GNUC_MINOR__ == 6)
+	// FIXME THREAD
 	static int lastBlock = 0;
-#else
-	thread_local int lastBlock = 0;
-#endif
 
 	// "clever reset"
 	if (c < 0x7f)