Autotools: Fix Autoconf syntax (erroneous dnl) (php#14987)

M4 interprets the dnl in this combination of m4_ifnblank as part of the
preceding text so the [] can be used to avoid this issue.

Author: petk

build/php.m4

@@ -377,7 +377,7 @@ AC_DEFUN([PHP_EVAL_LIBLINE],
     ;;
     esac
   done
-m4_ifnblank([$3], [m4_ifnblank([$2], [ext_shared=$_php_ext_shared_saved])])dnl
+m4_ifnblank([$3], [m4_ifnblank([$2], [ext_shared=$_php_ext_shared_saved])])[]dnl
 ])
 
 dnl