meson: Differentiate top-level and custom targets

meson: Differentiate top-level and custom targets

We need to create top-level targets to run targets with the ninja
command like `ninja <target_name>`.

Some targets (man, html, ...) have the same target name on both
top-level and custom target.  This creates a confusion for the meson
build:

$ meson compile -C build html

```
ERROR: Can't invoke target `html`: ambiguous name. Add target type
and/or path:
- ./doc/src/sgml/html:custom
- ./doc/src/sgml/html:alias
```

Solve that problem by adding '-custom' suffix to these problematic
targets' custom target names.  Top-level targets can be called with
both meson and ninja now:

$ meson compile -C build html
$ ninja -C build html

Author: Nazir Bilal Yavuz <byavuz81@gmail.com>
Suggested-by: Álvaro Herrera <alvherre@kurilemu.de>
Discussion: https://postgr.es/m/5508e572-79ae-4b20-84d0-010a66d077f2%40eisentraut.org