ffmpeg-v4l2-request: pkgrel=4 — use distcc shim when BUILDENV=distcc

FFmpeg's configure script resolves the compiler via `which gcc` and bakes
the absolute path /usr/bin/gcc into the generated makefiles. With
BUILDENV=(distcc ...) makepkg prepends /usr/lib/distcc/bin to PATH, but
configure has already resolved past the shim — so every translation unit
compiles locally despite the worker pool being available.

Pass --cc='distcc gcc' / --cxx='distcc g++' to configure when the distcc
shim path is present. Falls back to plain gcc/g++ otherwise so a !distcc
build is unaffected.

Validated locally: distcc-avahi 3.4-19 on fermi exposes
/usr/lib/distcc/bin/gcc -> ../../../bin/distcc, and the four-worker pool
(tesla, dcc1, dcc2, boltzmann) is reachable via DISTCC_HOSTS=+zeroconf.
Earlier ffmpeg builds on run 76 logged zero connections on any worker
despite distcc-avahi being correctly installed — this is the missing
piece.

arch/ffmpeg-v4l2-request-git/PKGBUILD

@@ -24,7 +24,7 @@ _srcname=FFmpeg
 _version='8.1'
 _commit='b57fbbe50c9b2656fad86a1a7eeabfd2b2a50935'  # v4l2-request-n8.1 tip 2026-04-24
 pkgver=8.1.r123329.b57fbbe
-pkgrel=3
+pkgrel=4
 epoch=2
 pkgdesc='FFmpeg with V4L2 Request API hwaccel (Rockchip / Allwinner stateless decode)'
 arch=('aarch64')
@@ -96,8 +96,20 @@ prepare() {
 build() {
   cd "${_srcname}"
 
+  # FFmpeg's configure resolves the compiler via `which` and bakes the
+  # absolute path into generated makefiles, bypassing the makepkg
+  # /usr/lib/distcc/bin shim. Pass it explicitly so `BUILDENV=(distcc ...)`
+  # actually distributes; otherwise everything compiles locally.
+  local _ffmpeg_cc=gcc _ffmpeg_cxx=g++
+  if [[ ":$PATH:" == *":/usr/lib/distcc/bin:"* ]]; then
+    _ffmpeg_cc='distcc gcc'
+    _ffmpeg_cxx='distcc g++'
+  fi
+
   ./configure \
     --prefix=/usr \
+    --cc="${_ffmpeg_cc}" \
+    --cxx="${_ffmpeg_cxx}" \
     --disable-debug \
     --disable-static \
     --disable-doc \