gh-115119: Detect _decimal dependencies using pkg-config

[erlend-aasland]

Only libmpdec 4.0.0 supports pkg-config.

• Issue: Unvendor libmpdec sources #115119

[erlend-aasland]

Tested on macOS with the following scenarios:

$ ./configure && make                         # Uses CPython vendored mpdecimal
$ ./configure --with-system-libmpdec && make  # pkg-config detects Homebrew installed libmpdec 4.0.0
$ LIBMPDEC_CFLAGS="-I$HOME/install" \
  LIBMPDEC_LIBS="-L$HOME/install -lm -lmpdec" \
  ./configure --with-system-libmpdec && make  # Build with custom libmpdec 2.5.1

For some reason the latter fails, but I suspect it is an issue with my setup.

[erlend-aasland]

cc. @ned-deily

[ned-deily]

LGTM, thanks! FWIW, I was able to successfully build and test with mpdecimal 2.5.1 as well as 4.0.0.

[erlend-aasland]

Thanks, Ned! I had forgotten about this :)