use autoconf macro to check for pkg-config (closes #15506)
diff --git a/configure.ac b/configure.ac
index 9698ea0..8177e27 100644
--- a/configure.ac
+++ b/configure.ac
@@ -2335,7 +2335,7 @@
 ],
 [AC_MSG_RESULT(no)])
 
-AC_PATH_TOOL([PKG_CONFIG], [pkg-config])
+PKG_PROG_PKG_CONFIG
 
 # Check for use of the system expat library
 AC_MSG_CHECKING(for --with-system-expat)