squish: Move to a module and split thirdparty lib

(cherry picked from commit 8311a78df5)

modules/squish/SCsub

@@ -0,0 +1,27 @@
+Import('env')
+Import('env_modules')
+
+env_squish = env_modules.Clone()
+
+# Thirdparty source files
+if (env["squish"] != "system"): # builtin
+	thirdparty_dir = "#thirdparty/squish/"
+	thirdparty_sources = [
+		"alpha.cpp",
+		"clusterfit.cpp",
+		"colourblock.cpp",
+		"colourfit.cpp",
+		"colourset.cpp",
+		"maths.cpp",
+		"rangefit.cpp",
+		"singlecolourfit.cpp",
+		"squish.cpp",
+	]
+
+	thirdparty_sources = [thirdparty_dir + file for file in thirdparty_sources]
+
+	env_squish.add_source_files(env.modules_sources, thirdparty_sources)
+	env_squish.Append(CPPPATH = [thirdparty_dir])
+
+# Godot source files
+env_squish.add_source_files(env.modules_sources, "*.cpp")