Fix build on modern toolchains (GCC 10+/15 C23, Clang, -fno-common)

[ptudor]

What

aprx doesn't build out of the box on current C toolchains. This series fixes four independent issues so that a clean ./configure && make succeeds on contemporary GCC, Clang, and cross-build systems. Each commit is one issue:

1. ttyreader: ANSI prototype for aprx_cfmakeraw() — the K&R definition leaves the 2nd parameter implicit-int. GCC 14, GCC 15 and Clang 19 promote -Wimplicit-int to an error by default, and C23 (GCC 15's default -std=gnu23) removes K&R definitions outright.
2. interface: rename local bool — bool/true/false are keywords in C23, so the parser's int bool; locals are a syntax error under GCC 15's default.
3. -fno-common multiple definitions — GCC 10+ and Clang 11+ default to -fno-common. aprx.h defines the pthread globals aprsis_thread/pthr_attrs without extern (so every translation unit that includes it emits its own copy), and now is defined in both timercmp.c and aprx-stat.c. Linking then fails with multiple definition of .... The fix declares the shared globals extern in aprx.h — exactly as is already done for the monotonic tick clock — and keeps a single owner for each. Behaviour is unchanged. This one bites every current compiler.
4. Link via $(CC) — Makefile.in sets LD=@CC@ and links with $(LD), so a default build links through the compiler driver. Build systems that set LD to the real linker on the make command line (OpenWrt, Buildroot, Yocto, ...) override that, and the link then runs ld directly and fails with ld: unrecognized option '-pthread'. Linking a threaded C program via the compiler driver is the portable, conventional form; it is a no-op for a default build where LD already expands to the compiler.

Tested — pristine master vs this branch

I built pristine master and then this branch on five platforms. "master build" = does upstream master compile+link as-is; "patched" = does this branch build and does aprx -V run.

Platform	Compiler	master build	fix(es) it needed	patched
Debian 12 (bookworm)	GCC 12.2	FAIL (link)	#3	builds, -V OK
Debian 13 (trixie)	GCC 14.2	FAIL (compile + link)	#1, #3	builds, -V OK
Fedora 43	GCC 15.2	FAIL (compile + link)	#1, #2, #3	builds, -V OK
FreeBSD 15.0	Clang 19.1	FAIL (compile + link)	#1, #3	builds, -V OK
OpenWrt (ramips/mt7620, musl)	GCC 15.2 cross	FAIL	#1, #2, #3, #4	builds, packaged + flashed

#3 breaks every platform. #1 breaks everything except GCC 12 (where implicit-int is still only a warning). #2 fires only where the compiler defaults to C23 (GCC 15). #4 fires only where the build system overrides LD (the OpenWrt cross build). Each commit is independent and can be taken on its own.

Out of scope (kept downstream)

Building against a musl sysroot additionally needs the kernel-AX.25 path (<netax25/ax25.h>) gated out, because musl ships no AX.25 headers. That is an environmental change — it would disable AX.25 on a normal glibc host that has libax25 — so it is kept in the downstream (OpenWrt) packaging rather than here. In the five builds above the AX.25 code path is exercised normally: the three glibc/Linux hosts have the AX.25 headers installed, and on FreeBSD PF_AX25 is undefined so aprx compiles that path out on its own.