build: use list for mutable retval rather than tuple

We define `retval` as a tuple and then replace the tuple by "appending"
items with `+=` but that actually creates a new tuple every time.
Because it is intended to be mutable, use a list instead, then return a
tuple from the function, as it should be immutable outside the function.

PR-URL: #41372
Reviewed-By: Christian Clauss <cclauss@me.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>

configure.py

@@ -828,7 +828,7 @@ def pkg_config(pkg):
   otherwise (None, None, None, None)"""
   pkg_config = os.environ.get('PKG_CONFIG', 'pkg-config')
   args = []  # Print pkg-config warnings on first round.
-  retval = ()
+  retval = []
   for flag in ['--libs-only-l', '--cflags-only-I',
                '--libs-only-L', '--modversion']:
     args += [flag]
@@ -843,9 +843,9 @@ def pkg_config(pkg):
     except OSError as e:
       if e.errno != errno.ENOENT: raise e  # Unexpected error.
       return (None, None, None, None)  # No pkg-config/pkgconf installed.
-    retval += (val,)
+    retval.append(val)
     args = ['--silence-errors']
-  return retval
+  return tuple(retval)
 
 
 def try_check_compiler(cc, lang):