gplugin/gplugin

Turn on -Wextra if it's available and clean the warnings it found.

2021-10-03, Gary Kramlich
c631d0391343
Turn on -Wextra if it's available and clean the warnings it found.

I also fixed most of the -Wpedantic warnings, but didn't turn it on as there
are a bunch of issues with it involving function pointers.

Testing Done:
Compiled and ran the unit tests.

Reviewed at https://reviews.imfreedom.org/r/989/
if not get_option('install-gplugin-query')
summary('gplugin-query',
'You have disabled installation of gplugin-query which is a very ' +
'useful diagnostics tool.',
section : 'Warnings')
endif
gplugin_query = executable('gplugin-query',
'gplugin-query.c',
dependencies : [gplugin_dep, GLIB, GOBJECT],
install : get_option('install-gplugin-query'))
if get_option('help2man')
custom_target('gplugin-query.1',
command : [help2man,
'--name=Query installed plugins', '--section=1',
'--help-option=--help-all', '--no-info',
'--output', '@OUTPUT@',
gplugin_query],
output : 'gplugin-query.1',
install : get_option('install-gplugin-query'),
install_dir : get_option('mandir') / 'man1')
endif