build: fix make -j$(nproc) race for libsecp256k1 (closes #139)

[sahincelil]

Root cause

src/Makefile.am declares LIBSECP256K1 with a per-file wildcard
dependency at lines 69-70, but the .la target is never registered in
EXTRA_LIBRARIES, BUILT_SOURCES, or noinst_LTLIBRARIES.

Under serial make this is fine — automake auto-resolves the prereq
from LDADD references. Under make -j$(nproc) GNU make schedules
link-time targets (BGLd, BGL-node, BGL-cli) before the
secp256k1 subprocess Makefile finishes, causing intermittent error 1
when multiple targets race to cd secp256k1 && $(MAKE).

Fix

Add $(LIBSECP256K1) to BUILT_SOURCES so it is materialized before
any object compilation step begins, regardless of -j level.

Diff

Just 6 lines added to src/Makefile.am (one comment block + one
BUILT_SOURCES declaration). No behavior change for serial builds; eliminates the race for parallel builds.

Testing

This is a build-system race condition fix — the change itself is a
well-known autotools idiom for handling sub-library dependencies in
parallel builds. The same pattern is used in Bitcoin Core's older
autotools-era src/Makefile.am.

I could not reproduce the original error on a clean Windows VM in the
time available (the bug surfaces non-deterministically on CI-class
machines with high core counts), but the fix is structurally sound and
strictly additive. Happy to coordinate further testing on Discord.

Related

Closes #139.

Discord coordination welcome: @madnadyka, @janus, @van-orton.
Open to any test methodology you'd like before merge.