configure: allow static builds on mingw

This patch is adopted from the MINGW-packages project. It makes it
possible to build curl both shared and static again.

URL: https://github.com/Alexpux/MINGW-packages/tree/master/mingw-w64-curl

m4/xc-lt-iface.m4

@@ -72,7 +72,7 @@ fi
 if test "x$xc_lt_want_enable_shared" = 'xyes' &&
   test "x$xc_lt_want_enable_static" = 'xyes'; then
   case $host_os in @%:@ (
-    mingw* | pw32* | cegcc* | os2* | aix*)
+    pw32* | cegcc* | os2* | aix*)
       xc_lt_want_enable_static='no'
       ;;
   esac