Fix fail to build with glibc 2.43

[sge-d-o]

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Pull request description

In Debian it was found glibc 2.43 comes with '#define _XOPEN_SOURCE 800' which causes the following errors:

In file included from utmp.c:31:
../../src/snoopy.h:31:10: error: '_XOPEN_SOURCE' redefined [-Werror]
31 | #define _XOPEN_SOURCE 700
| ^~~~~~~~~~~~~
In file included from /usr/include/utmp.h:21,
from utmp-snoopy.h:27,
from utmp.c:26:
/usr/include/features.h:234:10: note: this is the location of the previous definition
234 | # define _XOPEN_SOURCE 800
| ^~~~~~~~~~~~~

domain.c: In function 'snoopy_datasource_domain':
domain.c:126:17: error: assignment discards 'const' qualifier from pointer target type [-Werror=discarded-qualifiers]
126 | hashPtr = strchr(linePtr, '#');
| ^

This patch guards the '#define _XOPEN_SOURCE 700' in snoopy.h and drops the 'const' qualifier from the variable 'linePtr' resulting in building snoopy successfully using glibc 2.43.

[bostjan]

I suppose this is Debian 13 you're talking about here?

Looks sane, will merge.

BTW that const thing, that I'll need to double check that later, as it's quite likely that SonarCloud will complain about it.

[sge-d-o]

I suppose this is Debian 13 you're talking about here?

Actually this issue showed up while working on Debian 14. There is no problem with Debian 13, as this is the stable release which will not see an upgrade of glibc.

[sge-d-o]

To be clear, this PR is/was not about the packages you provide per https://a2o.github.io/snoopy-packages/ but about snoopy being packaged in Debian, see https://packages.debian.org/forky/snoopy. In this regard it is not related to #286.