Provide error on gcc < 4.1 when THREADSAFE
Provide a descriptive error message when compiling THREADSAFE on gcc
versions < 4.1. We require the atomic primitives (eg
`__sync_synchronize`) that were introduced in that version.
(Note, clang setes `__GNUC__` but appears to set its version > 4.1.)
diff --git a/src/thread-utils.h b/src/thread-utils.h
index f016198..2df2aeb 100644
--- a/src/thread-utils.h
+++ b/src/thread-utils.h
@@ -7,6 +7,12 @@
#ifndef INCLUDE_thread_utils_h__
#define INCLUDE_thread_utils_h__
+#if defined(__GNUC__) && defined(GIT_THREADS)
+# if (__GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ < 1))
+# error Atomic primitives do not exist on this version of gcc; configure libgit2 with -DTHREADSAFE=OFF
+# endif
+#endif
+
/* Common operations even if threading has been disabled */
typedef struct {
#if defined(GIT_WIN32)
