win32: provide macros for calling conventions

Provide more clarity for Win32 calling conventions - now that we always
compile to __cdecl on Win32, we'll define that as the the libgit2
calling convention.  Also offer NTAPI (__stdcall) calling conventions
for things that need callbacks from NTAPI code (eg fiber-local storage).

src/common.h

@@ -63,7 +63,9 @@
 #	include <pthread.h>
 #	include <sched.h>
 # endif
-#define GIT_STDLIB_CALL
+
+#define GIT_LIBGIT2_CALL
+#define GIT_SYSTEM_CALL
 
 #ifdef GIT_USE_STAT_ATIMESPEC
 # define st_atim st_atimespec

src/util.c

@@ -687,7 +687,7 @@ typedef struct {
 	void *payload;
 } git__qsort_r_glue;
 
-static int GIT_STDLIB_CALL git__qsort_r_glue_cmp(
+static int GIT_LIBGIT2_CALL git__qsort_r_glue_cmp(
 	void *payload, const void *a, const void *b)
 {
 	git__qsort_r_glue *glue = payload;

src/win32/msvc-compat.h

@@ -23,6 +23,14 @@ typedef SSIZE_T ssize_t;
 
 #endif
 
-#define GIT_STDLIB_CALL __cdecl
+/*
+ * Offer GIT_LIBGIT2_CALL for our calling conventions (__cdecl, always).
+ * This is useful for providing callbacks to userspace code.
+ *
+ * Offer GIT_SYSTEM_CALL for the system calling conventions (__stdcall on
+ * Win32).  Useful for providing callbacks to system libraries.
+ */
+#define GIT_LIBGIT2_CALL __cdecl
+#define GIT_SYSTEM_CALL NTAPI
 
 #endif