configure: avoid non-portable test == operator

[drhpc]

The single = works as well and also with shells other than bash
and friends.

Description

Target release

I would like my code to appear in release XXXXX

Type of contribution

• changes to code or doc authored by PLUMED developers, or additions of code in the core or within the default modules
• changes to a module not authored by you
• new module contribution or edit of a module authored by you

Copyright

• I agree to transfer the copyright of the code I have written to the PLUMED developers or to the author of the code I am modifying.

• the module I added or modified contains a COPYRIGHT file with the correct license information. Code should be released under an open source license. I also used the command cd src && ./header.sh mymodulename in order to make sure the headers of the module are correct.

Tests

• I added a new regtest or modified an existing regtest to validate my changes.
• I verified that all regtests are passed successfully on GitHub Actions.

[drhpc]

Also … I got a change in the pipeline where I need to ensure that the RPATH setting reaches the plumed main binary. It touches configure, too, so it would be nice if this simple one here could be merged first.

[GiovanniBussi]

Thanks! This looks like a portability fix, so I plan to backport it (manually) to version 2.7. Does it make sense?

Regarding this point, can you check if this is already fixed? I think I fixed it in v2.8 branch, so that it will end up in v2.8.1 (not released yet, though).

[carlocamilloni]

@GiovanniBussi is this or part of it still valid?