pass dependencies to toolchain.prepare when setting up build environment for extensions

[boegel]

fix for build problem with scipy in EESSI 2025.06, where EasyBuild is configured with search-path-cpp-headers set to include_paths

see also EESSI/software-layer#1216

$C_INCLUDE_PATH gets set to only paths of toolchain components, because list of dependencies is not passed to toolchain.prepare when build environment is set up for installing an extension

draft PR since I would like to come up with a test that catches this bug...

[boegel]

Test that verifies this fix has been added in e69baba

[bedroge]

@boegel Lgtm, only wondering if we should do the same test for the other header search options, as they are also affected (though the solution should solve it for all options)? Can we easily parameterize the test for --search-path-cpp-headers=[flags,cpath,include_paths] and check the value of respectively [CPPFLAGS, CPATH, C_INCLUDE_PATH / CPLUS_INCLUDE_PATH]?

[boegel]

@boegel Lgtm, only wondering if we should do the same test for the other header search options, as they are also affected (though the solution should solve it for all options)? Can we easily parameterize the test for --search-path-cpp-headers=[flags,cpath,include_paths] and check the value of respectively [CPPFLAGS, CPATH, C_INCLUDE_PATH / CPLUS_INCLUDE_PATH]?

@bedroge Good idea, that's indeed pretty easy to do, done in f9e1af1

[bedroge]

Lgtm!