Respect user CFLAGS.

--- kitty-0.15.0/setup.py
+++ kitty-0.15.0/setup.py
@@ -204,7 +204,7 @@
     if ccver >= (5, 0):
         df += ' -Og'
         float_conversion = '-Wfloat-conversion'
-    optimize = df if debug or sanitize else '-O3'
+    optimize = ''
     sanitize_args = get_sanitize_args(cc, ccver) if sanitize else set()
     cppflags = os.environ.get(
         'OVERRIDE_CPPFLAGS', '-D{}DEBUG'.format('' if debug else 'N'),
@@ -217,7 +217,7 @@
     cflags = os.environ.get(
         'OVERRIDE_CFLAGS', (
             '-Wextra {} -Wno-missing-field-initializers -Wall -Wstrict-prototypes -std=c11'
-            ' -pedantic-errors -Werror {} {} -fwrapv {} {} -pipe {} -fvisibility=hidden'
+            ' -pedantic-errors {} {} -fwrapv {} {} {} -fvisibility=hidden'
         ).format(
             float_conversion, std,
             optimize,
@@ -232,7 +232,7 @@
     )
     ldflags = os.environ.get(
         'OVERRIDE_LDFLAGS',
-        '-Wall ' + ' '.join(sanitize_args) + ('' if debug else ' -O3')
+        '-Wall ' + ' '.join(sanitize_args)
     )
     ldflags = shlex.split(ldflags)
     ldflags.append('-shared')
@@ -611,7 +611,7 @@


 def build_launcher(args, launcher_dir='.', bundle_type='source'):
-    cflags = '-Wall -Werror -fpie'.split()
+    cflags = '-Wall -fpie'.split()
     cppflags = []
     libs = []
     if args.profile or args.sanitize:
@@ -623,8 +623,6 @@
             cflags.append('-g')
         if args.profile:
             libs.append('-lprofiler')
-    else:
-        cflags.append('-O3')
     if bundle_type.endswith('-freeze'):
         cppflags.append('-DFOR_BUNDLE')
         cppflags.append('-DPYVER="{}"'.format(sysconfig.get_python_version()))