Add descriptive error message when pkg-config fails

The pkg_config helper for premake was not checking for errors. When it
failed to run, either when it is not installed or the queried package is
not found, a cryptic error message would be printed:

"Error: .../xenia/third_party/premake-core/src/base/string.lua:36: attempt to index a nil value (upvalue 's')"

Fix this by checking the return status when calling pkg-config and
printing a descriptive error message.

tools/build/scripts/pkg_config.lua

@@ -2,12 +2,21 @@
 
 pkg_config = {}
 
+local function pkg_config_call(lib, what)
+  local result, code = os.outputof("pkg-config --"..what.." "..lib)
+  if result then
+    return result
+  else
+    error("Failed to run 'pkg-config' for library '"..lib.."'. Are the development files installed?")
+  end
+end
+
 function pkg_config.cflags(lib)
   if not os.istarget("linux") then
     return
   end
   buildoptions({
-    ({os.outputof("pkg-config --cflags "..lib)})[1],
+    pkg_config_call(lib, "cflags"),
   })
 end
 
@@ -16,12 +25,12 @@ function pkg_config.lflags(lib)
     return
   end
   linkoptions({
-    ({os.outputof("pkg-config --libs-only-L "    ..lib)})[1],
-    ({os.outputof("pkg-config --libs-only-other "..lib)})[1],
+    pkg_config_call(lib, "libs-only-L"),
+    pkg_config_call(lib, "libs-only-other"),
   })
   -- 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]
+  local output = pkg_config_call(lib, "libs-only-l")
   for k, flag in next, string.explode(output, " ") do
     -- remove "-l"
     if flag ~= "" then