duplicates:

• SR-3948 Support Build Settings

Issue Description:

This is a follow up to SR-145 which concludes that include directories can be added via the pkgConfig setting. This is reasonable and works nicely if the library being exposed has such :-)

But when the library has no `.pc` file or the provided `.pc` file can't be processed by SPM (see SR-2481), it is desirable to include a custom `.pc` file in the SwiftPM system module itself.

Example:

import PackageDescription
let package = Package(
  name: "CApache",
  pkgConfig: "CApacheXcode",
)

Then I'd include CApacheXcode.pc in the package, e.g.:

prefix=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr
libdir=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include
includedir=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include
    
Name: CApache
Description: Apache compile flags for mod_swift
Version: 2.0.0
Libs: -L${libdir}  -lapr-1 -laprutil-1
Cflags: -I${includedir} -I${includedir}/apr-1 -I${includedir}/apache2

Summary: SPM should add the location of the cloned system module to the lookup list (PKG_CONFIG_PATH) for `pkgConfig` names, so that they can carry their own `.pc` files.