fix a mingw64 build on Linux with --enable-tcc64-mingw

* define CONFIG_WIN64=yes when ARCH=x86-64 (not CONFIG_WIN32=yes)
    * CONFIG_WIN64 now use a windows install part (not a Linux one)

lib/Makefile

@@ -68,7 +68,7 @@ else
 ifeq "$(TARGET)" "x86_64-win"
  OBJ = $(addprefix $(DIR)/,$(WIN64_O))
  TGT = -DTCC_TARGET_X86_64 -DTCC_TARGET_PE
- XCC = $(TCC) -B$(top_srcdir)/win32 -I$(top_srcdir)/include
+ XCC ?= $(TCC) -B$(top_srcdir)/win32 -I$(top_srcdir)/include
  XAR ?= $(DIR)/tiny_libmaker$(EXESUF)
  PICFLAGS =
 else