pcre subproject fixes: Allow fallback & static build

pcre subproject fix: Allow fallback to avoid libpcre being picked up from the build environment. Also build the built-in libpcre as static, as was done earlier.

Fix for the libpcre aspects of #3025 (closed)

Note: The static libpcre is still being installed together with glib. This isn't great, but it looks like this is managed by meson-specific pcre patches and I don't know how that ecosystem works. Ideally, static build shouldn't be installed.