Use the recommended $(...) instead of the obsolete backticks

Author: petk

build/php.m4

@@ -1964,8 +1964,8 @@ dnl Set installation PostgreSQL directory if given from configure argument.
     done
   ], [
     AC_MSG_RESULT([$PG_CONFIG])
-    PGSQL_INCLUDE=`$PG_CONFIG --includedir`
-    PGSQL_LIBDIR=`$PG_CONFIG --libdir`
+    PGSQL_INCLUDE=$($PG_CONFIG --includedir)
+    PGSQL_LIBDIR=$($PG_CONFIG --libdir)
   ])
 
   AS_IF([test -z "$PGSQL_INCLUDE" || test -z "PGSQL_LIBDIR"], [found_pgsql=no], [