handle file paths to libraries

Typically pkgconfig files specify cflags for linking with -L and -l,
however, pkgconfig files can also specify paths to library files. For
example, building Qt5 statically on macOS generates the following
pkgconfig file. Notice the absolute path to libqtpcre.a in Libs.private:

prefix=/Users/be/qt5-installed
exec_prefix=${prefix}
libdir=${prefix}/lib
includedir=${prefix}/include

host_bins=${prefix}/bin
qt_config=debug_and_release release debug build_all c++11 c++14 c++17 c++1z concurrent dbus no-pkg-config reduce_exports release_tools static stl

Name: Qt5 Core
Description: Qt Core module
Version: 5.15.5
Libs: -L${libdir} -lQt5Core
Libs.private: -framework DiskArbitration -framework IOKit -lm -framework AppKit -framework Security -framework ApplicationServices -framework CoreServices -framework CoreFoundation -framework Foundation -lz /Users/be/sw/qt-everywhere-src-5.15.5/qtbase/lib/libqtpcre2.a
Cflags: -DQT_CORE_LIB -I${includedir}/QtCore -I${includedir}

Building Qt5 statically on macOS with vcpkg generates this pkgconfig
file which has a handful of file paths for libraries:

prefix=${pcfiledir}/../..

exec_prefix=${prefix}
libdir=${prefix}/lib
includedir=${prefix}/include/qt5

host_bins=${prefix}/tools/qt5/bin
qt_config=release c++11 c++14 c++17 c++1z concurrent dbus no-pkg-config reduce_exports static stl properties animation textcodec big_codecs codecs itemmodel proxymodel concatenatetablesproxymodel textdate datestring doubleconversion filesystemiterator filesystemwatcher gestures identityproxymodel library mimetype process statemachine regularexpression settings sharedmemory sortfilterproxymodel stringlistmodel systemsemaphore temporaryfile translation transposeproxymodel xmlstream xmlstreamreader xmlstreamwriter

Name: Qt5 Core
Description: Qt Core module
Version: 5.15.3

Libs: -L"${libdir}" -lQt5Core  -L"${prefix}/lib" -L"${prefix}/lib/manual-link" -framework DiskArbitration -framework IOKit -lm -framework AppKit -framework Security -framework ApplicationServices -framework CoreServices -framework CoreFoundation -framework Foundation ${prefix}/lib/libz.a -ldouble-conversion ${prefix}/lib/libicui18n.a ${prefix}/lib/libicutu.a ${prefix}/lib/libicuuc.a ${prefix}/lib/libicuio.a ${prefix}/lib/libicudata.a ${prefix}/lib/libpcre2-16.a -lzstd  ${prefix}/lib/libbz2.a ${prefix}/lib/libpng16.a ${prefix}/lib/libicui18n.a ${prefix}/lib/libicutu.a ${prefix}/lib/libicuuc.a ${prefix}/lib/libicuio.a ${prefix}/lib/libicudata.a ${prefix}/lib/libzstd.a
Cflags: -DQT_CORE_LIB -I"${includedir}/QtCore" -I"${includedir}"

Cargo.toml

@@ -13,5 +13,6 @@ Cargo build scripts.
 """
 keywords = ["build-dependencies"]
 
-[dev-dependencies]
+[dependencies]
 lazy_static = "1"
+regex = "1"

src/lib.rs

@@ -568,6 +568,19 @@ impl Library {
     }
 
     fn parse_libs_cflags(&mut self, name: &str, output: &[u8], config: &Config) {
+        #[cfg(all(windows, target_env = "msvc"))]
+        lazy_static::lazy_static! {
+            static ref LIB_BASENAME: regex::Regex = regex::Regex::new(r"^(.*)\.(lib|dll)$").unwrap();
+        }
+        #[cfg(all(windows, target_env = "gnu"))]
+        lazy_static::lazy_static! {
+            static ref LIB_BASENAME: regex::Regex = regex::Regex::new(r"^lib(.*)\.(a|dll)$").unwrap();
+        }
+        #[cfg(not(windows))]
+        lazy_static::lazy_static! {
+            static ref LIB_BASENAME: regex::Regex = regex::Regex::new(r"^lib(.*)\.(a|so|dylib)").unwrap();
+        }
+
         let mut is_msvc = false;
         if let Ok(target) = env::var("TARGET") {
             if target.contains("msvc") {
@@ -669,7 +682,28 @@ impl Library {
                         self.include_paths.push(PathBuf::from(inc));
                     }
                 }
-                _ => (),
+                _ => {
+                    let path = std::path::Path::new(part);
+                    if path.is_file() {
+                        // Cargo doesn't have a means to directly specify a file path to link,
+                        // so split up the path into the parent directory and library name.
+                        if let Some(dir) = path.parent() {
+                            let link_search = format!("rustc-link-search={}", dir.display());
+                            config.print_metadata(&link_search);
+                        }
+                        if let Some(file_name) = path.file_name() {
+                            // libQt5Core.a becomes Qt5Core
+                            // libQt5Core.so.5 becomes Qt5Core
+                            if let Some(captures) =
+                                LIB_BASENAME.captures(&file_name.to_string_lossy())
+                            {
+                                let lib_basename = captures.get(1).unwrap().as_str();
+                                let link_lib = format!("rustc-link-lib={}", lib_basename);
+                                config.print_metadata(&link_lib);
+                            }
+                        }
+                    }
+                }
             }
         }