Add extra logging to debug the event loop

This should make tracking down the root cause of the
event loop pauses on X11 easier. And the infrastructure
should come in handy in the future as well.

setup.py

@@ -178,7 +178,8 @@ class Env:
 
 
 def init_env(
-    debug=False, sanitize=False, native_optimizations=True, profile=False
+    debug=False, sanitize=False, native_optimizations=True, profile=False,
+    extra_logging=()
 ):
     native_optimizations = native_optimizations and not sanitize and not debug
     cc, ccver = cc_version()
@@ -200,6 +201,8 @@ def init_env(
         )
     )
     cppflags = shlex.split(cppflags)
+    for el in extra_logging:
+        cppflags.append('-DDEBUG_{}'.format(el.upper().replace('-', '_')))
     cflags = os.environ.get(
         'OVERRIDE_CFLAGS', (
             '-Wextra -Wno-missing-field-initializers -Wall -std=c11'
@@ -508,7 +511,7 @@ def build(args, native_optimizations=True):
     compilation_database = {
         (k['file'], k.get('output')): k['arguments'] for k in compilation_database
     }
-    env = init_env(args.debug, args.sanitize, native_optimizations, args.profile)
+    env = init_env(args.debug, args.sanitize, native_optimizations, args.profile, args.extra_logging)
     try:
         compile_c_extension(
             kitty_env(), 'kitty/fast_data_types', args.incremental, compilation_database, all_keys, *find_c_files()
@@ -842,6 +845,13 @@ def option_parser():  # {{{
         default='lib',
         help='The name of the directory inside --prefix in which to store compiled files. Defaults to "lib"'
     )
+    p.add_argument(
+        '--extra-logging',
+        action='append',
+        choices=('event-loop',),
+        help='Turn on extra logging for debugging in this build. Can be specified multiple times, to turn'
+        'on different types of logging.'
+    )
     return p
 # }}}