Make it possible to even try building the OpenGL transitions for Windows

Change-Id: Id9b688a22518aedb8959120f909832f0cd84b324

configure.ac

@@ -867,8 +867,7 @@ AC_ARG_ENABLE(cairo-canvas,
 
 AC_ARG_ENABLE(opengl,
     AS_HELP_STRING([--disable-opengl],
-        [Determines whether to build the OpenGL 3D slide transitions component.]),
-,enable_opengl=yes)
+        [Determines whether to build the OpenGL 3D slide transitions component.]))
 
 AC_ARG_ENABLE(dbus,
     AS_HELP_STRING([--disable-dbus],
@@ -9702,14 +9701,32 @@ dnl ===================================================================
 AC_MSG_CHECKING([whether to build the OpenGL Transitions component])
 ENABLE_OPENGL=
 
-if test "$_os" = "Darwin"; then
+if test -z "$enable_opengl"; then
+    if test $_os = WINNT; then
+        # By default disable the OpenGL transitions for Windows (the code doesn't even compile
+        # currently). In other words, use --enable-opengl only if you plan to hack on that code for
+        # Windows.
+        enable_opengl=no
+    else
+        enable_opengl=yes
+    fi
+fi
+
+if test "x$enable_opengl" = "xno"; then
+    AC_MSG_RESULT([no])
+elif test "$_os" = "Darwin"; then
     # We use frameworks on Mac OS X, no need for detail checks
     ENABLE_OPENGL=TRUE
     SYSTEM_MESA_HEADERS=YES
     AC_MSG_RESULT([yes])
+elif test $_os = WINNT; then
+    # Experimental: try to use OpenGL on Windows
+    ENABLE_OPENGL=TRUE
+    # We need the internal "Mesa" headers.
+    SYSTEM_MESA_HEADERS=NO
+    BUILD_TYPE="$BUILD_TYPE MESA"
+    AC_MSG_RESULT([yes])
 else
-    if test "x$enable_opengl" != "xno"; then
-        if test "$_os" != "WINNT"; then
     save_LDFLAGS=$LDFLAGS
     LDFLAGS="$LDFLAGS -lm"
     AC_MSG_RESULT([yes])
@@ -9738,12 +9755,6 @@ else
         SYSTEM_MESA_HEADERS=NO
         BUILD_TYPE="$BUILD_TYPE MESA"
     fi
-        else
-            AC_MSG_RESULT([not on Windows])
-        fi
-    else
-       AC_MSG_RESULT([no])
-    fi
 fi
 
 AC_SUBST(SYSTEM_MESA_HEADERS)