[vte] build: Treat precious variables correctly

[Christian Persch <chpe src gnome org>]

commit fb3dd7cd9fb236f8e42e00bec84d64f832a9c3df
Author: Christian Persch <chpe gnome org>
Date:   Sun Apr 24 13:08:13 2016 +0200

    build: Treat precious variables correctly
    
    Test if a precious variable is set already before overwriting it.

 configure.ac |   12 ++++++++----
 1 files changed, 8 insertions(+), 4 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 62f0187..281438f 100644
--- a/configure.ac
+++ b/configure.ac
@@ -383,15 +383,19 @@ AM_CONDITIONAL([HAVE_VALA_0_31],[test "$have_vala_0_31" = "yes"])
 # ***************
 
 AC_ARG_VAR([GLIB_COMPILE_RESOURCES],[the glib-compile-resources programme])
-AC_PATH_PROG([GLIB_COMPILE_RESOURCES],[glib-compile-resources],[])
 if test -z "$GLIB_COMPILE_RESOURCES"; then
-  AC_MSG_ERROR([glib-compile-resources not found])
+  AC_PATH_PROG([GLIB_COMPILE_RESOURCES],[glib-compile-resources],[])
+  if test -z "$GLIB_COMPILE_RESOURCES"; then
+    AC_MSG_ERROR([glib-compile-resources not found])
+  fi
 fi
 
 AC_ARG_VAR([XMLLINT],[the xmllint programme])
-AC_PATH_PROG([XMLLINT],[xmllint],[])
 if test -z "$XMLLINT"; then
-  AC_MSG_ERROR([xmllint not found])
+  AC_PATH_PROG([XMLLINT],[xmllint],[])
+  if test -z "$XMLLINT"; then
+    AC_MSG_ERROR([xmllint not found])
+  fi
 fi
 
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