build: use case/esac instead of AS_CASE to avoid depending on a newer version of autoconf.
diff --git a/m4/gl.m4 b/m4/gl.m4
index bc67f6d..4a1bc48 100644
--- a/m4/gl.m4
+++ b/m4/gl.m4
@@ -62,12 +62,14 @@ LIBS="$PRELIBS"
AC_MSG_CHECKING([for GL library])
echo host=$host
-AS_CASE([$host],
- [*-mingw32], [GL_GL_LIBS="-lopengl32"
- GL_GLU_LIBS="-lglu32"],
- [GL_GL_LIBS="-lGL"
- GL_GLU_LIBS="-lGLU"]
-)
+case "x${host}" in
+ x*-mingw32) GL_GL_LIBS="-lopengl32"
+ GL_GLU_LIBS="-lglu32"
+ ;;
+ x*) GL_GL_LIBS="-lGL"
+ GL_GLU_LIBS="-lGLU"
+ ;;
+esac
if test "x$with_gl_lib" != "x" ; then
if test -d "$with_gl_lib" ; then