Statically link against the existing PCRE2 library as reported by pkg-config

[IceCodeNew]

By referring to the comments of pcre2-sys/build.rs, it appears to me that statically linking against an existing PCRE library is not supported yet.

rust-pcre2/pcre2-sys/build.rs

Lines 3 to 9 in 710307d

	// 1. Use the PCRE2 system library as reported by pkg-config if it exists
	// and only if we don't explicitly want a static build.
	// 2. Otherwise, statically build PCRE2 by hand.
	//
	// For step 1, we permit opting out of using the system library via either
	// explicitly setting the PCRE2_SYS_STATIC environment variable or if we
	// otherwise believe we want a static build (e.g., when building with MUSL).

I would like to ask for supporting linking ripgrep to a PCRE static library.

Here is my use case: I built HAProxy, ripgrep, and other stuff inside a docker image.
As the HAProxy was built with the latest PCRE2 library statically bounded, I naturally expect that the ripgrep could be bounded with the same library to cut off the compilation time and benefit from bumping of dependencies.

[BurntSushi]

Seems reasonable to me as long as it is easy to maintain. I will not be adding this functionality myself though.