Option for an LGPL-2.1+ FFmpeg build: drop gmp/--enable-version3, allow securetransport for TLS #87

Title: Option for an LGPL-2.1+ FFmpeg build: drop gmp/--enable-version3, allow securetransport for TLS

Hi, and thanks for the quick fix in #77 / #78. Removing --enable-nonfree made the regular artifacts redistributable. This is a follow-up from the same angle: shipping MPVKit in an App Store app.

What we see in 1.0.0

• Sources/BuildScripts/XCFrameworkBuild/main.swift still passes --enable-version3, and FFmpeg is built with gmp and gnutls (the dependency list near line 612).
• In FFmpeg n8.1.2's configure, gmp is the only enabled library in EXTERNAL_LIBRARY_VERSION3_LIST, so it is what forces --enable-version3. Inside FFmpeg, gmp only serves ffrtmpcrypt (RTMPE, encrypted RTMP).
• gnutls brings in nettle/hogweed and gmp, which are licensed LGPL-3.0-or-later OR GPL-2.0-or-later.

So today the regular build is effectively LGPLv3. For apps distributed on locked-down devices (iOS/tvOS App Store), LGPLv3's installation-information requirement for consumer products is hard to satisfy. LGPL-2.1+ is the more practical license there.

Proposal (opt-in, so nothing changes for current users)

A build option, say enableLGPLv21 (name up to you), that:

1. omits gmp and --enable-version3. The only feature lost is RTMPE.
2. omits gnutls (and with it nettle/hogweed/gmp) and uses FFmpeg's --enable-securetransport for TLS. That uses Apple's Security.framework, so no crypto library is bundled. A side benefit is simpler export compliance for App Store apps.
   • If Secure Transport is a problem (deprecated API on newer SDKs), the fallback is --enable-openssl with the OpenSSL 3.x already in the package. FFmpeg's configure accepts OpenSSL ≥ 3 without --enable-version3 when --enable-gpl is off.
3. optionally ships as separate products/artifacts, the way MPVKit-GPL sits beside MPVKit today.

Open questions we haven't verified:

• Does --enable-securetransport still build cleanly for iOS/tvOS with current Xcode?
• Does anything besides FFmpeg (libmpv, libbluray, libplacebo…) link against gnutls/nettle/gmp?

We're happy to test a branch on iOS and tvOS hardware, or to send a PR if you'd prefer that direction.