[Linux] Helper methods for pkg-config

- Fixes linking on GCC by putting libs in link group

tools/build/premake5.lua

@@ -3,5 +3,6 @@ require("vstudio")
 include("scripts/build_paths.lua")
 include("scripts/force_compile_as_c.lua")
 include("scripts/force_compile_as_cc.lua")
+include("scripts/pkg_config.lua")
 include("scripts/platform_files.lua")
 include("scripts/test_suite.lua")

tools/build/scripts/pkg_config.lua

@@ -0,0 +1,34 @@
+-- Helper methods to use the system pkg-config utility
+
+pkg_config = {}
+
+function pkg_config.cflags(lib)
+  if os.istarget("windows") then
+    return
+  end
+  buildoptions({
+    ({os.outputof("pkg-config --cflags "..lib)})[1],
+  })
+end
+
+function pkg_config.lflags(lib)
+  if os.istarget("windows") then
+    return
+  end
+  linkoptions({
+    ({os.outputof("pkg-config --libs-only-L "    ..lib)})[1],
+    ({os.outputof("pkg-config --libs-only-other "..lib)})[1],
+  })
+  -- We can't just drop the stdout of the `--libs` command in
+  -- linkoptions because library order matters
+  local output = ({os.outputof("pkg-config --libs-only-l "..lib)})[1]
+  for k, flag in next, string.explode(output, " ") do
+    -- remove "-l"
+    links(string.sub(flag, 3))
+  end
+end
+
+function pkg_config.all(lib)
+    pkg_config.cflags(lib)
+    pkg_config.lflags(lib)
+end