[gtksourceviewmm] Build: Use atkmm-2.30, not atkmm-2.26

[Murray Cumming <murrayc src gnome org>]

commit 989e986df4b7c7d1c109e1494e9700b2db190590
Author: Murray Cumming <murrayc murrayc com>
Date:   Fri Sep 27 22:41:16 2019 +0200

    Build: Use atkmm-2.30, not atkmm-2.26
    
    This is the version of atkmm currently used by gtkmm-4.0. Ideally we
    would not have to specify this directly.

 configure.ac | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/configure.ac b/configure.ac
index 5590958..8fd454d 100644
--- a/configure.ac
+++ b/configure.ac
@@ -32,7 +32,7 @@ PKG_CHECK_MODULES([GTKSOURCEVIEWMM], [$GTKSOURCEVIEWMM_MODULES])
 
 MM_PKG_CONFIG_SUBST([GTHREAD_CFLAGS], [--cflags-only-other gthread-2.0])
 MM_PKG_CONFIG_SUBST([GMMPROC_DIR], [--variable=gmmprocdir glibmm-2.64])
-MM_PKG_CONFIG_SUBST([GMMPROC_EXTRA_M4_DIR], [--variable=gmmprocm4dir pangomm-2.42 atkmm-2.26 gtkmm-4.0])
+MM_PKG_CONFIG_SUBST([GMMPROC_EXTRA_M4_DIR], [--variable=gmmprocm4dir pangomm-2.42 atkmm-2.30 gtkmm-4.0])
 
 MM_ARG_DISABLE_DEPRECATED_API([GTKSOURCEVIEWMM])