Always use GCC provided zlib

Always use the GCC provided zlib to make the compiler as independent of
the host system as possible.

source-builder/config/gcc-common-1.cfg

@@ -129,12 +129,6 @@ BuildRoot: %{_tmppath}/%{name}-root-%(%{__id_u} -n)
   languages="$languages,objc"
 %endif
 
-%if %{_host_os} == mingw32
-  zlib_option=""
-%else
-  zlib_option="--with-system-zlib"
-%endif
-
   %{host_build_flags}
 
   ../${source_dir_gcc}/configure \
@@ -152,7 +146,6 @@ BuildRoot: %{_tmppath}/%{name}-root-%(%{__id_u} -n)
     --disable-libstdcxx-pch \
     --with-gnu-as --with-gnu-ld --verbose \
     --with-newlib \
-    "$zlib_option" \
     --disable-nls --without-included-gettext \
     --disable-win32-registry \
     --enable-version-specific-runtime-libs \