add support for %(rpath_enabled)s template value

[boegel]

There are situations where we should be able to detect in an easyconfig file whether or not RPATH linking is used.

This can be leveraged in easyconfigs where pre-built binaries or libraries are included, like (for example):

postinstallcmds = [
    "if %(rpath_enabled)s; then patchelf %(installdir)s/...; fi",
]

[boegel]

One use case for this is NextPolish-1.4.1-GCCcore-12.3.0.eb, which includes pre-built libraries like nextpolish1.so which don't have an RPATH section, so the sanity check fails on them.
Rightfully so, because if $LD_LIBRARY_PATH is not set, they're relying on libraries provided by the OS, like /lib64/libbz2.so.1, /lib64/liblzma.so.5, and /lib64/libz.so.1.

[boegel]

I've re-targeted this to develop, since it would be useful short term in the context of easybuilders/easybuild-easyconfigs#21588

cc @jfgrimm

[jfgrimm]

as suggested in the conf call, this example could be shortened to:

postinstallcmds = [
    "if true; then patchelf %(installdir)s/...; fi",
]

if we set it to true or false

Alternatively, we could consider a different approach and do something more like the ARCH constant we have in easyconfigs? e.g. from UCX-1.15.0-GCCcore-13.2.0.eb

if ARCH == "riscv64":                                                                                                   
    patches += ['UCX-1.15.0-add_riscv_support.patch']                                                                   
    checksums += [{'UCX-1.15.0-add_riscv_support.patch':                                                                
                   '700640d469f441f3ee2c9d42e3a533f78f0c892c1d3c221aebd41279668d118e'}]                                 
    preconfigopts = 'autoreconf -fi && '

we could similarly have:

if USE_RPATH:
    postinstallcmds = ...

[boegel]

@jfgrimm The USE_RPATH constant is a good one, but doesn't work, because it's possible that the RPATH linking is being selectively disabled (through a hook for example) for a particular installation, so it's not a session-wide constant value (that's different from ARCH, which is fixed for the entire session since it's tied to the system you're on)

[jfgrimm]

@boegel fair enough, I had thought it might be awkward with the potential that people hook on a case-by-case basis