[libxml2] Fix build with older pkg-config versions

[Nick Wellnhofer <nwellnhof src gnome org>]

commit 3c4e4bb7264afeab0704df287343d4c77ca8f8a1
Author: Nick Wellnhofer <wellnhofer aevum de>
Date:   Fri Aug 5 16:24:51 2022 +0200

    Fix build with older pkg-config versions
    
    The PKG_CHECK_VAR macro was introduced in pkg-config 0.28. Guard the
    invocation with m4_ifdef.
    
    Fixes #374.

 configure.ac | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/configure.ac b/configure.ac
index b4564d71..1a684642 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1226,7 +1226,8 @@ else
         [have_libicu=no])
 
     if test "x$have_libicu" = "xyes"; then
-        PKG_CHECK_VAR([ICU_DEFS], [icu-i18n], [DEFS])
+        m4_ifdef([PKG_CHECK_VAR],
+            [PKG_CHECK_VAR([ICU_DEFS], [icu-i18n], [DEFS])])
         if test "x$ICU_DEFS" != "x"; then
             CPPFLAGS="$CPPFLAGS $ICU_DEFS"
         fi