Summary:
Fallback on trying the same filename as the actual file.
There were some regressions due to the former matching expression:
string(REGEX REPLACE "^(.*\n)?File=([^\n]+kcfg).*\n.*$" "\\2"  _kcfg_FILENAME "${_contents}")

This one would allow for the File field to be the kcfgc file (which doesn't
make sense).
With this patch we're not reproducing this behaviour, but we're allowing for the
things that aren't building to work. After all, most kcfg and kcfgc files share
the same name.

Reviewers: #frameworks, davidedmundson

Reviewed By: davidedmundson

Subscribers: aacid, kfunk

Differential Revision: https://phabricator.kde.org/D3226

Summary:
Instead of having an odd fake of it. Will help some required
refactorings and already showed some issues, fixed by this patch,
namely:
- don't use string(regex replace) to extract a string from another
string. in case it doesn't match it will offer the whole content which
is never what we want.
- messages(ERROR), the correct parameter is FATAL_ERROR, cmake
understands "ERROR" as mere output string
- turn the macro into a function, otherwise 2 calls in the same
subdirectory are dangerous.

CCBUG: 371562

Test Plan: tests still pass, projects that use the macro still build

Reviewers: #frameworks, dfaure

Reviewed By: dfaure

Subscribers: dfaure

Differential Revision: https://phabricator.kde.org/D3178

REVIEW: 129188

The test checks that saving a read-only config file fails. But because root can write into read-only files, the test is failing when running by root. Check for uid when running the test and make it XFAIL if we are running as root.

REVIEW: 128916