Fix "'__thread' not at start of declaration" warnings (-Wextra)

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>

api.doxygen

@@ -17,6 +17,7 @@ CASE_SENSE_NAMES      = NO
 
 PREDEFINED = \
   "GIT_EXTERN(x)=x" \
+  "GIT_EXTERN_TLS(x)=x" \
   "GIT_INLINE(x)=x" \
   "GIT_BEGIN_DECL=" \
   "GIT_END_DECL=" \

src/git/common.h

 #ifndef INCLUDE_git_common_h__
 #define INCLUDE_git_common_h__
 
+#include "thread-utils.h"
+
 #ifdef __cplusplus
 # define GIT_BEGIN_DECL  extern "C" {
 # define GIT_END_DECL    }
@@ -20,6 +22,16 @@
 # define GIT_EXTERN(type) extern type
 #endif
 
+/** Declare a public TLS symbol exported for application use. */
+#ifdef __GNUC__
+# define GIT_EXTERN_TLS(type) extern \
+                              __attribute__((visibility("default"))) \
+                              GIT_TLS \
+                              type
+#else
+# define GIT_EXTERN_TLS(type) extern GIT_TLS type
+#endif
+
 /** Declare a function as always inlined. */
 #if defined(_MSC_VER)
 # define GIT_INLINE(type) static __inline type

src/git/errors.h

@@ -2,7 +2,6 @@
 #define INCLUDE_git_errors_h__
 
 #include "common.h"
-#include "thread-utils.h"
 
 /**
  * @file git/errors.h
@@ -14,7 +13,7 @@ GIT_BEGIN_DECL
 
 /** The git errno. */
 #if defined(GIT_TLS)
-GIT_EXTERN(int) GIT_TLS git_errno;
+GIT_EXTERN_TLS(int) git_errno;
 
 #elif defined(GIT_HAS_PTHREAD)
 # define git_errno (*git__errno_storage())