Windows: Remove dependency on pthreadGC2.dll

Commit 44626dc7 (MSVC: Windows-native implementation for subset
of threads API, 2010-01-15) introduces builtin replacement of
pthreadGC2.dll functionality, thus we can completely drop
dependency on this dll.

Signed-off-by: Michael Lukashov <michael.lukashov@gmail.com>
Acked-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -994,6 +994,7 @@ ifeq ($(uname_S),Windows)
 	COMPAT_CFLAGS = -D__USE_MINGW_ACCESS -DNOGDI -DHAVE_STRING_H -DHAVE_ALLOCA_H -Icompat -Icompat/fnmatch -Icompat/regex -Icompat/fnmatch -Icompat/win32 -DSTRIP_EXTENSION=\".exe\"
 	BASIC_LDFLAGS = -IGNORE:4217 -IGNORE:4049 -NOLOGO -SUBSYSTEM:CONSOLE -NODEFAULTLIB:MSVCRT.lib
 	EXTLIBS = advapi32.lib shell32.lib wininet.lib ws2_32.lib
+	PTHREAD_LIBS =
 	lib =
 ifndef DEBUG
 	BASIC_CFLAGS += -GL -Os -MT
@@ -1036,11 +1037,12 @@ ifneq (,$(findstring MINGW,$(uname_S)))
 	NO_PYTHON = YesPlease
 	BLK_SHA1 = YesPlease
 	THREADED_DELTA_SEARCH = YesPlease
-	COMPAT_CFLAGS += -D__USE_MINGW_ACCESS -DNOGDI -Icompat -Icompat/fnmatch
+	COMPAT_CFLAGS += -D__USE_MINGW_ACCESS -DNOGDI -Icompat -Icompat/fnmatch -Icompat/win32
 	COMPAT_CFLAGS += -DSTRIP_EXTENSION=\".exe\"
 	COMPAT_OBJS += compat/mingw.o compat/fnmatch/fnmatch.o compat/winansi.o \
 		compat/win32/pthread.o
 	EXTLIBS += -lws2_32
+	PTHREAD_LIBS =
 	X = .exe
 ifneq (,$(wildcard ../THIS_IS_MSYSGIT))
 	htmldir=doc/git/html/