gdb: Build with system readline and use MinGW compilers.

Select MinGW compilers for Windows builds on Windows.

Use the system readline rather than GDB's which does not
handle cmd.exe correctly.

source-builder/config/gdb-7-1.cfg

@@ -7,10 +7,14 @@
 #
 # See if the simulator has been disabled for Windows.
 #
+# Build using the system's readline, it is in better shape
+# than GDB's one with MSYS2.
+#
 %if %{_host_os} == win32
  %if %{defined win32-gdb-disable-sim}
   %define gdb-disable-sim 1
-  %endif
+ %endif
+ %define with_system_readline 1
 %endif
 
 #
@@ -107,6 +111,7 @@ BuildRoot: %{_tmppath}/%{name}-root-%(%{__id_u} -n)
     --build=%{_build} --host=%{_host} \
     --target=%{_target} \
     --verbose --disable-nls \
+    %{?with_system_readline:--with-system-readline} \
     --without-included-gettext \
     --disable-win32-registry \
     --disable-werror \