mysterious preprocessor flag in compile line

[Nico Schlömer <nico.schloemer@gmail.com>]

Hi everyone,

I'm trying to build a package for waybar [1], and I'm running some
test builds on a PPA. I'm inching close, but now there's something I
cannot figure out: The build process adds the flag
```
-DONLY_C_LOCALE=
```
to the compile line, and that makes the build fail in the date.h
dependency [2]. (It should be -DONLY_C_LOCALE=0 or -DONLY_C_LOCALE=1.)

Any idea where `-DONLY_C_LOCALE=` might come from and how to remove it?

Cheers,
Nico

[1] https://github.com/Alexays/Waybar
[2] https://launchpadlibrarian.net/488841716/buildlog_ubuntu-focal-amd64.waybar_0.9.2-202007161952-0968218e-1focal1_BUILDING.txt.gz