Patch to make the target libraries 'configure' scripts find the proper CPP.
I noticed that building the mingw32 cross compiler.
Looking at the build script for mingw in archlinux, I think that only nixos
needs this patch. I don't know why.
diff --git a/Makefile.in b/Makefile.in
index 93f66b6..d691917 100644
--- a/Makefile.in
+++ b/Makefile.in
@@ -266,6 +266,7 @@ BASE_TARGET_EXPORTS = \
 	AR="$(AR_FOR_TARGET)"; export AR; \
 	AS="$(COMPILER_AS_FOR_TARGET)"; export AS; \
 	CC="$(CC_FOR_TARGET) $(XGCC_FLAGS_FOR_TARGET) $$TFLAGS"; export CC; \
+	CPP="$(CC_FOR_TARGET) $(XGCC_FLAGS_FOR_TARGET) $$TFLAGS -E"; export CC; \
 	CFLAGS="$(CFLAGS_FOR_TARGET)"; export CFLAGS; \
 	CONFIG_SHELL="$(SHELL)"; export CONFIG_SHELL; \
 	CPPFLAGS="$(CPPFLAGS_FOR_TARGET)"; export CPPFLAGS; \
@@ -291,11 +292,13 @@ BASE_TARGET_EXPORTS = \
 RAW_CXX_TARGET_EXPORTS = \
 	$(BASE_TARGET_EXPORTS) \
 	CXX_FOR_TARGET="$(RAW_CXX_FOR_TARGET)"; export CXX_FOR_TARGET; \
-	CXX="$(RAW_CXX_FOR_TARGET) $(XGCC_FLAGS_FOR_TARGET) $$TFLAGS"; export CXX;
+	CXX="$(RAW_CXX_FOR_TARGET) $(XGCC_FLAGS_FOR_TARGET) $$TFLAGS"; export CXX; \
+	CXXCPP="$(RAW_CXX_FOR_TARGET) $(XGCC_FLAGS_FOR_TARGET) $$TFLAGS -E"; export CXX;
 
 NORMAL_TARGET_EXPORTS = \
 	$(BASE_TARGET_EXPORTS) \
-	CXX="$(CXX_FOR_TARGET) $(XGCC_FLAGS_FOR_TARGET) $$TFLAGS"; export CXX;
+	CXX="$(CXX_FOR_TARGET) $(XGCC_FLAGS_FOR_TARGET) $$TFLAGS"; export CXX; \
+	CXXCPP="$(CXX_FOR_TARGET) $(XGCC_FLAGS_FOR_TARGET) $$TFLAGS -E"; export CXX;
 
 # Where to find GMP
 HOST_GMPLIBS = @gmplibs@