Some mingw cross-compile support use something like:

gmake DONT_CLOBBER_CC=yes CC=mingw32-gcc

src/build/Common.gmake

@@ -191,6 +191,11 @@ else
 endif
 BUILDPATH	?=	$(BUILD)
 
+gcc_machine		:= $(shell ${CC} -dumpmachine)
+ifeq ($(gcc_machine),mingw32)
+ os		:= Win32
+endif
+
 # Get OS
 ifndef os
  os		:=	$(shell uname)
@@ -281,9 +286,7 @@ else
 endif
 
 # OS Specific Flags
-ifeq ($(os),sunos)    # Solaris
- CFLAGS	+= -D__solaris__ -DNEEDS_DAEMON -DNEEDS_FORKPTY -DNEEDS_CFMAKERAW
- # Solaris 10 provides setenv()
+ifeq ($(os),sunos)    # Solaris CFLAGS	+= -D__solaris__ -DNEEDS_DAEMON -DNEEDS_FORKPTY -DNEEDS_CFMAKERAW # Solaris 10 provides setenv()
  ifeq ($(shell if [ `uname -r | sed 's/\.//'` -lt 510 ] ; then echo "Yes" ; else echo "No" ; fi),Yes)
   CFLAGS += -DNEEDS_SETENV
  endif
@@ -336,9 +339,14 @@ else
       MT_CFLAGS    += -DUSE_XP_SEMAPHORES
       # This makes ctime_r() be the correct one.
       #CFLAGS	   += -D_POSIX_PTHREAD_SEMANTICS
-     else            # Linux / Other UNIX
-      XP_SEM :=    1
-      MT_CFLAGS    += -DUSE_XP_SEMAPHORES
+     else
+      ifeq ($(os),win32) # Windows
+       CFLAGS		+= -D_WIN32 -D_WIN32_WINNT=0x0500 -DWINVER=0x0500 -D_WIN32_IE=0x0500
+       MT_CFLAGS	+= -D_WIN32
+      else           # Linux / Other UNIX
+       XP_SEM :=    1
+       MT_CFLAGS    += -DUSE_XP_SEMAPHORES
+      endif
      endif
     endif
    endif

src/conio/Common.gmake

@@ -21,31 +21,36 @@ ifeq ($(os),openbsd)
  CFLAGS += -DHAVE_VASPRINTF
 endif
 
-ifdef USE_SYSTEM_CURSES
- CIOLIB-MT_LIBS		+=	$(UL_PRE)curses$(UL_SUF)
-else
- CIOLIB-MT_LIBS		+=	$(UL_PRE)ncurses$(UL_SUF)
-endif
-ifndef NO_X
- ifdef STATIC
-  CIOLIB-MT_LIBS		+=	-L$(X_PATH)$(DIRSEP)lib -lX11
+ifneq ($(os),win32)
+ ifdef USE_SYSTEM_CURSES
+  CIOLIB-MT_LIBS		+=	$(UL_PRE)curses$(UL_SUF)
  else
-  ifeq ($(os),darwin)
-   ifeq ($(shell uname -r),6.8)
+  CIOLIB-MT_LIBS		+=	$(UL_PRE)ncurses$(UL_SUF)
+ endif
+ ifndef NO_X
+  ifdef STATIC
+   CIOLIB-MT_LIBS		+=	-L$(X_PATH)$(DIRSEP)lib -lX11
+  else
+   ifeq ($(os),darwin)
+    ifeq ($(shell uname -r),6.8)
+     CIOLIB-MT_LIBS		+=  $(UL_PRE)dl$(UL_SUF)
+    endif
+   endif
+   ifeq ($(os),linux)
+    CIOLIB-MT_LIBS		+=  $(UL_PRE)dl$(UL_SUF)
+   endif
+   ifeq ($(os),sunos)
     CIOLIB-MT_LIBS		+=  $(UL_PRE)dl$(UL_SUF)
    endif
   endif
-  ifeq ($(os),linux)
-   CIOLIB-MT_LIBS		+=  $(UL_PRE)dl$(UL_SUF)
-  endif
-  ifeq ($(os),sunos)
-   CIOLIB-MT_LIBS		+=  $(UL_PRE)dl$(UL_SUF)
-  endif
+ else
+   ifeq ($(os),darwin)
+    CIOLIB-MT_CFLAGS		+=  -DDISABLE_X11=1
+   endif
  endif
 else
-  ifeq ($(os),darwin)
-   CIOLIB-MT_CFLAGS		+=  -DDISABLE_X11=1
-  endif
+ NO_X			 :=  1
+ CIOLIB-MT_CFLAGS        +=  -DDISABLE_X11=1
 endif
 
 # Find SDL headers!

src/conio/GNUmakefile

@@ -4,7 +4,9 @@ include $(SRC_ROOT)/build/Common.gmake	# defines clean and output directory rule
 
 CFLAGS += $(XPDEV-MT_CFLAGS) $(CIOLIB-MT_CFLAGS)
 
-OBJS	+=	$(MTOBJODIR)$(DIRSEP)curs_cio$(OFILE)
+ifneq ($(os),win32)
+ OBJS	+=	$(MTOBJODIR)$(DIRSEP)curs_cio$(OFILE)
+endif
 ifdef NO_X
  CFLAGS	+=	-DNO_X
 else

src/syncterm/GNUmakefile

@@ -119,18 +119,26 @@ else
 endif
 INSTALL_DATA	?=	install -m 0444
 
-ifneq ($(os),haiku)
- OBJS	+= $(MTOBJODIR)$(DIRSEP)comio_nix$(OFILE) \
+ifeq ($(os),win32)
+ OBJS	+= $(MTOBJODIR)$(DIRSEP)comio_win32$(OFILE) \
            $(MTOBJODIR)$(DIRSEP)modem$(OFILE) \
            $(MTOBJODIR)$(DIRSEP)comio$(OFILE)
+else
+ ifneq ($(os),haiku)
+  OBJS	+= $(MTOBJODIR)$(DIRSEP)comio_nix$(OFILE) \
+           $(MTOBJODIR)$(DIRSEP)modem$(OFILE) \
+           $(MTOBJODIR)$(DIRSEP)comio$(OFILE)
+ endif
 endif
 OBJS += $(MTOBJODIR)$(DIRSEP)conn_pty$(OFILE)
 ifndef bcc
  ifneq ($(os),sunos)
   ifneq ($(os),darwin)
-    ifneq ($(os),haiku)
+   ifneq ($(os),haiku)
+    ifneq ($(os),win32)
       EXTRA_LIBS   +=  -lutil
-	endif
+    endif
+   endif
   endif
  endif
 endif

src/xpdev/GNUmakefile

@@ -4,7 +4,9 @@ include $(SRC_ROOT)/build/Common.gmake
 ifdef XP_SEM
 	MTOBJS	+=	$(MTOBJODIR)$(DIRSEP)xpsem$(OFILE)
 endif
-MTOBJS	+=	$(MTOBJODIR)$(DIRSEP)xpevent$(OFILE)
+ifneq ($(os),win32)
+ MTOBJS	+=	$(MTOBJODIR)$(DIRSEP)xpevent$(OFILE)
+endif
 
 CFLAGS	+=	-DSOUNDCARD_H_IN=$(shell if [ -f /usr/include/sys/soundcard.h ] ; then echo 1 ; elif [ -f /usr/include/soundcard.h ] ; then echo 2 ; elif [ -f /usr/include/linux/soundcard.h ] ; then echo 3 ; else echo 0 ; fi) -I. $(XPDEV_CFLAGS)
 ifeq ($(shell if [ -f /usr/include/alsa/asoundlib.h ] ; then echo YES ; fi),YES)