try to document c_stdlib_version in knowledge base

[minrk]

I tried to write down my understanding of the consequences of using a newer c_stdlib_version than the default, since this seems to be tripping up some folks (myself included). Please correct anything that's not right.

The gist I tried to get across:

• c_stdlib_version is not a transitive dependency, but you need to make sure $LDFLAGS are respected. Mention -Wl,--allow-shlib-undefined by name.
• old c_stdlib_version 2.12 is still the default both within conda-forge and for users at runtime. c-compiler/gcc_linux-64 set $LDFLAGS on activation, not gcc, which is relevant for user compilation outside conda-build.

Related issues:

• https://github.com/conda-forge/linux-sysroot-feedstock/issues/63
• https://github.com/conda-forge/openmpi-feedstock/issues/143

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[minrk]

Relevant section in preview

[jaimergp]

@h-vetinari if you have spare cycle, could you help review this? I'm afraid I lack the expertise 😬 Thanks! 🙏

[h-vetinari]

@h-vetinari if you have spare cycle, could you help review this? I'm afraid I lack the expertise 😬 Thanks! 🙏

I think this PR (and the docs generally) need an update now that cos7 has become the default. It's still reasonable to keep the content the same (modulo the exact name of the missing symbols), because the situation will be the same with 2.17 -> 2.28