archivers/liblz4: fix multithreading and tests

• Add -pthread to CFLAGS/LDFLAGS and the needed #define

Reported by: diizzy@

• Fix a bogus shell variable expansion using $$l when it should use $l
• Fix hard-coded "make" with GNU Makefiles BINARY_ALIAS it to ${GMAKE}
• For now, prevent Python 3.14 for tests, until we fully resolve the circular dependency. Let's investigate if we can unlink lz4 from the zstd build (with its maintainer).

PR: 251183
PR: 290732
Approved by: portmgr@ (just-fix-it blanket)