[libxml2] Make configure.ac work with older pkg-config

[Nick Wellnhofer <nwellnhof src gnome org>]

commit 91d576de8b890f9a9f86b0d0c711edd3a8ed90a7
Author: Nick Wellnhofer <wellnhofer aevum de>
Date:   Tue Apr 9 13:16:50 2019 +0200

    Make configure.ac work with older pkg-config
    
    Older versions of pkg.m4 require the action-if-not-found argument of
    the PKG_CHECK_MODULES macro to be non-empty. Use a colon (null command)
    instead of an empty string.
    
    Fixes #50.

 configure.ac | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 9cae50c8..be575944 100644
--- a/configure.ac
+++ b/configure.ac
@@ -396,7 +396,7 @@ else
         # Try pkg-config first so that static linking works.
         PKG_CHECK_MODULES([Z],[zlib],
             [WITH_ZLIB=1],
-            [ ])
+            [:])
     fi
 
     if test "$WITH_ZLIB" = "0"; then
@@ -435,7 +435,7 @@ else
         # Try pkg-config first so that static linking works.
         PKG_CHECK_MODULES([LZMA],[liblzma],
             [WITH_LZMA=1],
-            [ ])
+            [:])
     fi
 
     # If pkg-config failed, fall back to AC_CHECK_LIB. This