Switch the 21 standard modules from the defs.mak srcdir derivation to
configure-provided substitution (same template as tiles):
srcdir = @srcdir@
VPATH = @srcdir@
MAGICDIR = @top_builddir@
defs.mak's dual-mode guard sees `srcdir` already set (origin=file) and
skips the srcdir derivation for these modules.
Verified out-of-tree: config.status writes srcdir=<src>/<mod> into each
build/<mod>/Makefile; `make modules libs` builds all 33 module libraries
(lib*.o + lib*.a, rc=0).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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