Fix link ordering so libm/pthread resolve in make all

[stumpylog]

Problem

On Linux, make all fails at link time:

undefined reference to `sqrt' / `pow' / `ceil'

-lm lives in CFLAGS, which is expanded before the archive inputs on each link line. GNU ld --as-needed (Debian/Ubuntu default) then drops libm before it reaches sqlite-vec.a. Same trap for libpthread (sqlite is built threadsafe but -lpthread is never linked).

Fixes #84, #93, #2.

Fix

Move link libraries out of CFLAGS into a dedicated LINK_LIBS appended after the inputs on every link line.

-CFLAGS += -lm
+LINK_LIBS += -lm -ldl -lpthread

Verification

make clean && make all                              # was failing, now exits 0
dist/sqlite3 :memory: "SELECT vec_version()"        # v0.1.10-alpha.4

Complementary to #290/#292, which add $(CFLAGS) to compile lines but don't touch the -lm link ordering.