Don't use the system zlib

This doesn't work on all our target platforms (Windows, for example).

Makefile.in

@@ -153,7 +153,6 @@ stamps/build-gcc-linux-stage1: $(srcdir)/riscv-gcc stamps/build-binutils-linux \
 		--without-headers \
 		--disable-shared \
 		--disable-threads \
-		--with-system-zlib \
 		--enable-tls \
 		--enable-languages=c \
 		--disable-libatomic \
@@ -182,7 +181,6 @@ stamps/build-gcc-linux-stage2: $(srcdir)/riscv-gcc $(addprefix stamps/build-glib
 		$(CONFIGURE_HOST) \
 		--prefix=$(INSTALL_DIR) \
 		--with-sysroot=$(SYSROOT) \
-		--with-system-zlib \
 		--enable-shared \
 		--enable-tls \
 		--enable-languages=c,c++,fortran \
@@ -223,7 +221,6 @@ stamps/build-gcc-newlib-stage1: $(srcdir)/riscv-gcc stamps/build-binutils-newlib
 		--disable-threads \
 		--disable-tls \
 		--enable-languages=c,c++ \
-		--with-system-zlib \
 		--with-newlib \
 		--disable-libmudflap \
 		--disable-libssp \
@@ -264,7 +261,6 @@ stamps/build-gcc-newlib-stage2: $(srcdir)/riscv-gcc stamps/build-newlib
 		--disable-shared \
 		--disable-threads \
 		--enable-languages=c,c++ \
-		--with-system-zlib \
 		--enable-tls \
 		--with-newlib \
 		--with-headers=$(INSTALL_DIR)/riscv$(XLEN)-unknown-elf/include \