[gtksourceview] build: use PKG_CHECK_VAR

[Sébastien Wilmet <swilmet src gnome org>]

commit e38e7f34ef9c91d0c57b204415731d234aaec546
Author: Sébastien Wilmet <swilmet gnome org>
Date:   Sun May 7 18:13:19 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 2666a6f..1fc5cb6 100644
--- a/configure.ac
+++ b/configure.ac
@@ -218,8 +218,8 @@ 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])
 
 # Vala
 AX_REQUIRE_DEFINED([VAPIGEN_CHECK])