Add GODOT_CPP_PATH and use pkg-config for dependencies

SConstruct

@@ -1,10 +1,15 @@
 #!/usr/bin/env python
+from SCons import __version__ as scons_raw_version
 import os
 import sys
 
 ext_name = "xlib"
 
-env = SConscript("godot-cpp/SConstruct")
+# Load the environment from godot-cpp
+godot_cpp_path = "godot-cpp"
+if 'GODOT_CPP_PATH' in os.environ:
+    godot_cpp_path = os.environ['GODOT_CPP_PATH']
+env = SConscript(godot_cpp_path + "/SConstruct")
 
 # For the reference:
 # - CCFLAGS are compilation flags shared between C and C++
@@ -19,11 +24,13 @@ env.Append(CPPPATH=["src/"])
 sources = Glob("src/*.cpp")
 
 # Include dependency libraries for the extension
-env.Append(LIBS=["X11", "XRes", "Xtst"])
-
+if 'PKG_CONFIG_PATH' in os.environ:
+    env['ENV']['PKG_CONFIG_PATH'] = os.environ['PKG_CONFIG_PATH']
+env.ParseConfig("pkg-config x11 --cflags --libs")
+env.ParseConfig("pkg-config xres --cflags --libs")
+env.ParseConfig("pkg-config xtst --cflags --libs")
 
 # Generating the compilation DB (`compile_commands.json`) requires SCons 4.0.0 or later.
-from SCons import __version__ as scons_raw_version
 
 scons_ver = env._get_major_minor_revision(scons_raw_version)
 if scons_ver < (4, 0, 0):
@@ -39,7 +46,8 @@ env.Alias("compiledb", env.CompilationDatabase())
 
 # Build the shared library
 library = env.SharedLibrary(
-    "addons/{}/bin/lib{}{}{}".format(ext_name, ext_name, env["suffix"], env["SHLIBSUFFIX"]),
+    "addons/{}/bin/lib{}{}{}".format(ext_name,
+                                     ext_name, env["suffix"], env["SHLIBSUFFIX"]),
     source=sources,
 )