build: tiles/Makefile.in uses @srcdir@ substitution

Switch tiles from the defs.mak srcdir *derivation* to configure-provided
substitution:
  srcdir   = @srcdir@
  VPATH    = @srcdir@
  MAGICDIR = @top_builddir@
defs.mak's dual-mode guard now sees `srcdir` already set (origin=file) and
skips the $(abspath)/$(patsubst) srcdir computation for this module.

Verified: config.status writes srcdir=<src>/tiles into build/tiles/Makefile;
make -C tiles module compiles from that srcdir (libtiles.o, rc=0) with the
derivation bypassed.  This is the template the remaining modules follow.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tiles/Makefile.in

@@ -3,7 +3,11 @@
 #
 
 MODULE    = tiles
-MAGICDIR  = ..
+# Provided by configure: the module's source dir + the build top.
+# defs.mak sees `srcdir` already set and skips its own srcdir derivation.
+srcdir    = @srcdir@
+VPATH     = @srcdir@
+MAGICDIR  = @top_builddir@
 SRCS      = tile.c search.c
 
 include ${MAGICDIR}/defs.mak