build-sys: meson: Process subdirectories before generating configuration

Subdirectories add to the top-level cdata (specifically, the SIMD
detection happens in the pulsecore meson.build), so we were missing
HAVE_MMX/SSE2/NEON defines without this fix.
2 jobs for master in 6 minutes and 40 seconds (queued for 2 minutes and 39 seconds)
Status Job ID Name Coverage
  Build
passed #537613
build-autotools

00:05:13

passed #537614
build-meson

00:01:27