support: allow override of ADAFLAGS, LDFLAGS, library soversion

[asarhaddon]

All this would be useful when packaging the support library for Debian.
I have tried to copy the code from gnatcoll so that it also benefits windows and macos.
Thanks for considering.

[CLAassistant]

All committers have signed the CLA.

[pmderodat]

Thank you for your contributions! In order to merge them, we need you to sign the CLA.

Regarding the commit messages: could you keep them wrapped at 76 columns?

[pmderodat]

We would like to keep things working with default options: please replace "windows" with "" here and adjust the rest of the patch to avoid defining Library_Version in that case?

[pmderodat]

Thank you. Unfortunately, this does not work:

$ gprbuild -Psupport/langkit_support.gpr
langkit_support.gpr:16:04: no value defined for "os"
gprbuild: "support/langkit_support.gpr" processing failed

I suppose this is a bug in GPRbuild, sorry I didn’t check earlier. To workaround it, I would suggest to replace "" with "unknown" or similar.

[raph-amiard]

@asarhaddon ping! We would love to merge this, but you need to answer @pmderodat 's last comment

[asarhaddon]

Hello.
I had a more simple idea meanwhile.
Instead of concatenating a suffix depending on LANGKIT_SUPPORT_OS and LANGKIT_SUPPORT_SOVERSION into Library_Version, we should maybe support a single LANGKIT_SONAME variable.
If the variable is unset, Library_Version remains unset in order to keep the existing behaviour. If it is set, the content is assigned to Library_Version.
Both usage and implementation would be simplified.
Should I implement this idea or an "unknown" choice for LANGKIT_SUPPORT_OS?

[pmderodat]

[…] we should maybe support a single LANGKIT_SONAME variable. If the variable is unset, Library_Version remains unset in order to keep the existing behaviour. If it is set, the content is assigned to Library_Version

This looks like a good idea. Yes, let’s do it, thank you!

[pmderodat]

Hello @asarhaddon, and sorry for taking so much time to get back to you.

The patch looks almost good to me: could you just rename the LANGKIT_SONAME external to LANGKIT_SUPPORT_SONAME?

Thank you in advance,