[gtef] build: use PKG_CHECK_VAR

[Sébastien Wilmet <swilmet src gnome org>]

commit 3616dbd18b13b4383d1ca89044341eb5f34e26e7
Author: Sébastien Wilmet <swilmet gnome org>
Date:   Sun May 7 18:07:57 2017 +0200

    build: use PKG_CHECK_VAR

 configure.ac |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 6d12361..91125d1 100644
--- a/configure.ac
+++ b/configure.ac
@@ -134,8 +134,8 @@ AX_CODE_COVERAGE
 AX_REQUIRE_DEFINED([AX_VALGRIND_CHECK])
 AX_VALGRIND_CHECK
 
-GLIB_PREFIX="`$PKG_CONFIG --variable=prefix glib-2.0`"
-AC_SUBST([GLIB_PREFIX])
+AX_REQUIRE_DEFINED([PKG_CHECK_VAR])
+PKG_CHECK_VAR([GLIB_PREFIX], [glib-2.0], [prefix])
 
 # Output files
 AC_CONFIG_FILES([