Porting MinGW multithreading bugfix to makefile.mingw

Original fix:
https://github.com/bitcoin/bitcoin/commit
/8864019f6d88b13d3442843d9e6ebeb8dd938831

src/makefile.mingw

@@ -43,7 +43,7 @@ LIBS= \
  -l ssl \
  -l crypto
 
-DEFS=-DWIN32 -D_WINDOWS -DBOOST_THREAD_USE_LIB -DBOOST_SPIRIT_THREADSAFE
+DEFS=-D_MT -DWIN32 -D_WINDOWS -DBOOST_THREAD_USE_LIB -DBOOST_SPIRIT_THREADSAFE
 DEBUGFLAGS=-g
 CFLAGS=-mthreads -O2 -w -Wall -Wextra -Wformat -Wformat-security -Wno-unused-parameter $(DEBUGFLAGS) $(DEFS) $(INCLUDEPATHS)
 # enable: ASLR, DEP and large address aware
@@ -63,7 +63,7 @@ ifneq (${USE_IPV6}, -)
 	DEFS += -DUSE_IPV6=$(USE_IPV6)
 endif
 
-LIBS += -l kernel32 -l user32 -l gdi32 -l comdlg32 -l winspool -l winmm -l shell32 -l comctl32 -l ole32 -l oleaut32 -l uuid -l rpcrt4 -l advapi32 -l ws2_32 -l mswsock -l shlwapi
+LIBS += -l mingwthrd -l kernel32 -l user32 -l gdi32 -l comdlg32 -l winspool -l winmm -l shell32 -l comctl32 -l ole32 -l oleaut32 -l uuid -l rpcrt4 -l advapi32 -l ws2_32 -l mswsock -l shlwapi
 
 # TODO: make the mingw builds smarter about dependencies, like the linux/osx builds are
 HEADERS = $(wildcard *.h)