meson: fix build warnings

[CoffeeFlux]

In particular, there are a lot of new ODR warnings. Determine if these are a problem and whether they're Meson-specific, and ideally fix them.

[milahu]

slightly related: libiconv should not be searched on linux

on linux, it always says

Library iconv found: NO

i solved this with

# Aegisub/meson.build

iconv_found = false
iconv_dep = []
if host_machine.system() != 'linux'
    # darwin has libiconv. windows?
    iconv_dep = cc.find_library('iconv', required: false)
    iconv_found = iconv_dep.found()
endif
if not (iconv_found or cc.has_function('iconv_open')) # linux has iconv_open in libc
    iconv_sp = subproject('iconv') # this really needs to be replaced with a proper port
    iconv_dep = iconv_sp.get_variable('libiconv_dep')
endif
deps += iconv_dep