C unity builds are not reproducible if the macro __FILE__ is used

**Describe the bug**
Unity builds of C source files are not reproducible if the macro \_\_FILE\_\_ is used

**To Reproduce**
Here's a minimal Meson project exhibiting the issue.
`meson.build` content:

project('repro', 'c', default_options: ['unity=on'])
executable('repro', sources: 'repro.c')

`repro.c` content:

#include
int main()
{
printf("%s", __FILE__);
}

**Expected behavior**
Building the example project should create an executable named `repro` which, when executed, prints

../repro.c

The actual generated executable prints the full path, rendering the build not reproducible.

**System parameters**
I verified the issue with the following system parameters:

* OS: Xubuntu 20.04
* Meson versions: 0.54.1 and 0.54.2
* Compilers: clang 10 and gcc 9.3
* Python version: 3.8.2
* Ninja version: 1.10.0

Start with the minimal project in meson.build and repro.c, then reproduce the unity build using the listed Meson, compiler, and Ninja versions. Trace how the unity build handles __FILE__. Done means the generated repro executable prints ../repro.c rather than the full source path.

Written by the indexing model from the issue text.