configure: Automatically select GTK+ 3.0 if GTK+ 2.0 is unavailable

The configure option --with-gtkabi=3.0 is still supported, but no longer
needed when GTK+-2.0 is missing. When no GTK+ ABI is selected by the
user, configure first tries 2.0, then 3.0.

For some platforms (e.g. Windows) newer binaries of GTK+ are only
available for GTK+ 3.0. Now building on these platforms is a little bit
easier.
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

configure

@@ -317,7 +317,7 @@ glusterfs_discard="no"
 glusterfs_zerofill="no"
 virtio_blk_data_plane=""
 gtk=""
-gtkabi="2.0"
+gtkabi=""
 vte=""
 tpm="no"
 libssh2=""
@@ -1970,6 +1970,18 @@ fi
 ##########################################
 # GTK probe
 
+if test "$gtkabi" = ""; then
+    # The GTK ABI was not specified explicitly, so try whether 2.0 is available.
+    # Use 3.0 as a fallback if that is available.
+    if $pkg_config --exists "gtk+-2.0 >= 2.18.0"; then
+        gtkabi=2.0
+    elif $pkg_config --exists "gtk+-3.0 >= 3.0.0"; then
+        gtkabi=3.0
+    else
+        gtkabi=2.0
+    fi
+fi
+
 if test "$gtk" != "no"; then
     gtkpackage="gtk+-$gtkabi"
     if test "$gtkabi" = "3.0" ; then
@@ -1983,7 +1995,7 @@ if test "$gtk" != "no"; then
         libs_softmmu="$gtk_libs $libs_softmmu"
         gtk="yes"
     elif test "$gtk" = "yes"; then
-        feature_not_found "gtk" "Install gtk2 or gtk3 (requires --with-gtkabi=3.0 option to configure) devel"
+        feature_not_found "gtk" "Install gtk2 or gtk3 devel"
     else
         gtk="no"
     fi
@@ -2006,7 +2018,11 @@ if test "$vte" != "no"; then
         libs_softmmu="$vte_libs $libs_softmmu"
         vte="yes"
     elif test "$vte" = "yes"; then
-        feature_not_found "vte" "Install libvte or libvte-2.90 (requires --with-gtkabi=3.0 option to configure) devel"
+        if test "$gtkabi" = "3.0"; then
+            feature_not_found "vte" "Install libvte-2.90 devel"
+        else
+            feature_not_found "vte" "Install libvte devel"
+        fi
     else
         vte="no"
     fi