unset CC and CXX before glibc build

if the user set them for earlier stages
we don't want to use those

Makefile.in

@@ -138,7 +138,7 @@ stamps/build-glibc-linux-headers: src/glibc stamps/build-gcc-linux-stage1
 	rm -rf $@ $(notdir $@)
 	mkdir $(notdir $@)
 	mkdir -p $(SYSROOT)/usr/lib $(SYSROOT)/lib
-	cd $(notdir $@) && $(CURDIR)/$</configure \
+	cd $(notdir $@) && CC= CXX= $(CURDIR)/$</configure \
 		--host=riscv$(XLEN)-unknown-linux-gnu \
 		--prefix=/usr \
 		libc_cv_forced_unwind=yes \
@@ -154,7 +154,7 @@ stamps/build-glibc-linux$(XLEN)$(MULTILIB): src/glibc stamps/build-gcc-linux-sta
 	rm -rf $@ $(notdir $@)
 	mkdir $(notdir $@)
 	mkdir -p $(SYSROOT)/usr/lib $(SYSROOT)/lib
-	cd $(notdir $@) && CFLAGS="$(CFLAGS_FOR_TARGET) -g -O2" \
+	cd $(notdir $@) && CC= CXX= CFLAGS="$(CFLAGS_FOR_TARGET) -g -O2" \
 		ASFLAGS="$(ASFLAGS_FOR_TARGET)" \
 		$(CURDIR)/$</configure \
 		--host=riscv$(XLEN)-unknown-linux-gnu \