Enable compilation database for Scons

This is necessary for interfacing with external tools such as clang-tidy

.gitignore

@@ -127,6 +127,7 @@ wesnoth.plg
 *.exe
 *.dll
 *.so
+compile_commands.json
 
 # library files
 .libs

SConstruct

@@ -120,6 +120,7 @@ opts.AddVariables(
     BoolVariable("OS_ENV", "Forward the entire OS environment to scons", False),
     BoolVariable("history", "Clear to disable GNU history support in lua console", True),
     BoolVariable('force_color', 'Always produce ANSI-colored output (GNU/Clang only).', False),
+    BoolVariable('compile_db', 'Produce a compile_commands.json file.', False),
     )
 
 #
@@ -189,6 +190,11 @@ if env['distcc']:
 
 if env['ccache']: env.Tool('ccache')
 
+if env['compile_db']:
+    env.Tool('compilation_db')
+    cdb = env.CompilationDatabase()
+    Alias('cdb', cdb) 
+
 boost_version = "1.67"
 
 def SortHelpText(a, b):