Autotools: Fix phpdbg build (#15373)

With 04a67cd the list of source files
are now added alphabetically. Previously the phpdbg_parser.c was added
before the phpdbg_lexer.c. Which caused the
"sapi/phpdbg/phpdbg_lexer.l:8:10: fatal error: 'phpdbg_parser.h' file
not found" error.

To make the order of source files irrelevant, the Makefile substitutions
needs to be fixed - the 3rd argument of PHP_ADD_MAKEFILE_FRAGMENT macro,
which is the substitution of the $(builddir) Make variable. The
$(builddir)/phpdbg_lexer.lo was previously substituted to an absolute
path. And the relative should be used, for Make to be able to find the
dependent target.

Author: petk

sapi/phpdbg/config.m4

@@ -64,7 +64,7 @@ if test "$PHP_PHPDBG" != "no"; then
 
   PHP_ADD_MAKEFILE_FRAGMENT([$abs_srcdir/sapi/phpdbg/Makefile.frag],
     [$abs_srcdir/sapi/phpdbg],
-    [$abs_builddir/sapi/phpdbg])
+    [sapi/phpdbg])
   PHP_SELECT_SAPI([phpdbg],
     [program],
     m4_normalize([