threads: Cleanup TLS declarations

This time for good.

include/git2/thread-utils.h

@@ -31,41 +31,48 @@
  *          http://predef.sourceforge.net/precomp.html
  */
 
-#define GIT_HAS_TLS 1
+#ifdef GIT_THREADS
+#  define GIT_HAS_TLS 1
 
-#if defined(__APPLE__) && defined(__MACH__)
+/* No TLS in Cygwin */
+#  if defined(__CHECKER__) || defined(__CYGWIN__)
+#    undef GIT_HAS_TLS
+#    define GIT_TLS
+
+/* No TLS in Mach binaries for Mac OS X */
+#  elif defined(__APPLE__) && defined(__MACH__)
 #    undef GIT_TLS
 #    define GIT_TLS
 
-#elif defined(__GNUC__) || \
+/* Normal TLS for GCC */
+#  elif defined(__GNUC__) || \
         defined(__SUNPRO_C) || \
         defined(__SUNPRO_CC) || \
         defined(__xlc__) || \
         defined(__xlC__)
 #    define GIT_TLS __thread
 
-#elif defined(__INTEL_COMPILER)
+/* ICC may run on Windows or Linux */
+#  elif defined(__INTEL_COMPILER)
 #    if defined(_WIN32) || defined(_WIN32_CE)
 #      define GIT_TLS __declspec(thread)
 #    else
 #      define GIT_TLS __thread
 #    endif
 
-#elif defined(_WIN32) || \
+/* Declspec for MSVC in Win32 */
+#  elif defined(_WIN32) || \
         defined(_WIN32_CE) || \
         defined(__BORLANDC__)
 #    define GIT_TLS __declspec(thread)
 
-#else
+/* Other platform; no TLS */
+#  else
 #    undef GIT_HAS_TLS
 #    define GIT_TLS /* nothing: tls vars are thread-global */
-#endif
-
-/* sparse and cygwin don't grok thread-local variables */
-#if defined(__CHECKER__) || defined(__CYGWIN__)
-# undef GIT_HAS_TLS
-# undef GIT_TLS
+#  endif
+#else /* Disable TLS if libgit2 is not threadsafe */
 #  define GIT_TLS
-#endif
+#endif /* GIT_THREADS */
 
 #endif /* INCLUDE_git_thread_utils_h__ */