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}"

src/lib.rs

@@ -669,7 +669,14 @@ impl Library {
                         self.include_paths.push(PathBuf::from(inc));
                     }
                 }
-                _ => (),
+                _ => {
+                    let path = std::path::Path::new(part);
+                    if path.is_file() {
+                        // Pass file paths directly to the linker instead of going through
+                        // rustc adding platform-specific prefixes/suffixes with rustc-link-lib.
+                        config.print_metadata(&format!("rustc-link-arg={}", part));
+                    }
+                }
             }
         }