Contribs: srt, don't link to pthreadGC2 on Mingw

(cherry picked from commit e9ed3a579b)

Signed-off-by: Steve Lhomme <robux4@ycbcr.xyz>

contrib/src/srt/0004-cmake-pthread-win32.patch

@@ -0,0 +1,11 @@
+--- srt/CMakeLists.txt.old	2018-07-11 22:41:30.169381997 +0200
++++ srt/CMakeLists.txt	2018-07-11 22:41:40.324958847 +0200
+@@ -338,7 +338,7 @@
+ endif()
+ 
+ if (MINGW)
+-	set (PTHREAD_LIBRARY -lpthreadGC2)
++	set (PTHREAD_LIBRARY -lpthread)
+ 	# XXX CONSIDER THIS. It came from a merged previous master version
+ 	# and was under an if(NOT MINGW)...elseif(THREADS_FOUND) condition.
+ 	# Might be that the current form still suffices; resolve this and remove comment.

contrib/src/srt/rules.mak

@@ -26,6 +26,7 @@ srt: srt-$(SRT_VERSION).tar.gz .sum-srt
 	$(APPLY) $(SRC)/srt/0001-api-Don-t-use-inet_ntop.patch
 	$(APPLY) $(SRC)/srt/0002-win32-Only-include-inttypes.h-with-MSVC.patch
 	$(APPLY) $(SRC)/srt/0003-cmake-Only-install-Windows-headers-in-win-subdir.patch
+	$(APPLY) $(SRC)/srt/0004-cmake-pthread-win32.patch
 	mv srt-$(SRT_VERSION) $@ && touch $@
 
 .srt: srt toolchain.cmake