enabling gtk vclplugs is supposed to enforce requiring system cairo

dropped by...

commit 3d1f28dd1fbb2fe3f5b933a9d692fb4d033f08b6
Date:   Sun May 30 18:19:40 2021 +0200

    configure: Refactor platform defaults

so cairo and pixman get downloaded and built out of the box with --enable-gtk3

Change-Id: I5f0b05ccaca008b9a36c6456811c9e105f13e334
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/116550
Tested-by: Jenkins
Reviewed-by: Jan-Marek Glogowski <glogow@fbihome.de>
diff --git a/configure.ac b/configure.ac
index fbd13a3..dcb01d4 100644
--- a/configure.ac
+++ b/configure.ac
@@ -11532,6 +11532,7 @@ if test "$test_gtk3" = yes -a "x$enable_gtk3" = "xyes" -o "x$enable_gtk3_kde5" =
    if test "$with_system_cairo" = no; then
        add_warning 'Non-system cairo combined with gtk3 is assumed to cause trouble; proceed at your own risk.'
    fi
    : ${with_system_cairo:=yes}
    PKG_CHECK_MODULES(GTK3, gtk+-3.0 >= 3.20 gtk+-unix-print-3.0 gmodule-no-export-2.0 glib-2.0 >= 2.38 cairo)
    GTK3_CFLAGS=$(printf '%s' "$GTK3_CFLAGS" | sed -e "s/-I/${ISYSTEM?}/g")
    GTK3_CFLAGS="$GTK3_CFLAGS -DGDK_DISABLE_DEPRECATED -DGTK_DISABLE_DEPRECATED"
@@ -11554,6 +11555,7 @@ if test "test_gtk4" = yes -a "x$enable_gtk4" = "xyes"; then
    if test "$with_system_cairo" = no; then
        add_warning 'Non-system cairo combined with gtk4 is assumed to cause trouble; proceed at your own risk.'
    fi
    : ${with_system_cairo:=yes}
    PKG_CHECK_MODULES(GTK4, gtk4 gmodule-no-export-2.0 glib-2.0 >= 2.38 cairo atk)
    GTK4_CFLAGS=$(printf '%s' "$GTK4_CFLAGS" | sed -e "s/-I/${ISYSTEM?}/g")
    GTK4_CFLAGS="$GTK4_CFLAGS -DGDK_DISABLE_DEPRECATED -DGTK_DISABLE_DEPRECATED"
@@ -13106,6 +13108,7 @@ dnl ===================================================================
if test "$test_cairo" = "yes"; then
    AC_MSG_CHECKING([whether to use the system cairo])

    : ${with_system_cairo:=$with_system_libs}
    if test "$with_system_cairo" = "yes"; then
        SYSTEM_CAIRO=TRUE
        AC_MSG_RESULT([yes])