mingw/msvc: use the new-style RUNTIME_PREFIX helper

This change also allows us to stop overriding argv[0] with the absolute
path of the executable, allowing us to preserve e.g. the case of the
executable's file name.

This fixes #1496 partially.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

compat/mingw.c

@@ -3453,7 +3453,7 @@ void mingw_startup(void)
 		die_startup();
 
 	/* determine size of argv and environ conversion buffer */
-	maxlen = wcslen(_wpgmptr);
+	maxlen = wcslen(wargv[0]);
 	for (i = 1; i < argc; i++)
 		maxlen = max(maxlen, wcslen(wargv[i]));
 	for (i = 0; wenv[i]; i++)
@@ -3473,8 +3473,7 @@ void mingw_startup(void)
 	buffer = malloc_startup(maxlen);
 
 	/* convert command line arguments and environment to UTF-8 */
-	__argv[0] = wcstoutfdup_startup(buffer, _wpgmptr, maxlen);
-	for (i = 1; i < argc; i++)
+	for (i = 0; i < argc; i++)
 		__argv[i] = wcstoutfdup_startup(buffer, wargv[i], maxlen);
 	for (i = 0; wenv[i]; i++)
 		environ[i] = wcstoutfdup_startup(buffer, wenv[i], maxlen);

config.mak.uname

@@ -395,6 +395,7 @@ ifeq ($(uname_S),Windows)
 	# SNPRINTF_RETURNS_BOGUS = YesPlease
 	NO_SVN_TESTS = YesPlease
 	RUNTIME_PREFIX = YesPlease
+	HAVE_WPGMPTR = YesWeDo
 	NO_ST_BLOCKS_IN_STRUCT_STAT = YesPlease
 	USE_WIN32_MMAP = YesPlease
 	MMAP_PREVENTS_DELETE = UnfortunatelyYes
@@ -563,6 +564,7 @@ ifneq (,$(findstring MINGW,$(uname_S)))
 	NO_MKDTEMP = YesPlease
 	NO_SVN_TESTS = YesPlease
 	RUNTIME_PREFIX = YesPlease
+	HAVE_WPGMPTR = YesWeDo
 	NO_ST_BLOCKS_IN_STRUCT_STAT = YesPlease
 	USE_WIN32_MMAP = YesPlease
 	MMAP_PREVENTS_DELETE = UnfortunatelyYes