Enable specification of krb5-config via environment variable

[neirbowj]

Closes #250

[neirbowj]

Build look good with py36, py37, py38 (default), and py39 on FreeBSD 12.2-RELEASE amd64 against:

• Heimdal in base
• Heimdal in ports
• MIT in ports

[frozencemetery]

Hmm, okay. I think I had a perhaps more invasive change in mind.

Right now we've got $GSSAPI_LINKER_ARGS (overrides link_args, but can be derived from krb5_config), $GSSAPI_COMPILER_ARGS (overrides compile_args, but can be derived from krb5_config), and $GSSAPI_MAIN_LIB (overrides main_lib, but can be derived from link_args and platform goo). So what I had in mind was:

1. The freebsd stanza should respect link_args and compile_args if they're already set - this allows for proper overriding with the existing env vars.
2. Optionally, some of the code in setup.py could move around to make more sense, or move into helper functions for clarity. I think classes is probably overkill, though.
3. As an optional add-on, we add a variable for the location of krb5_config. This is then used to derive link_args and compile_args if they're not set (and the it makes sense on the platform etc.).

Since I'm being fussy here, please let me know if you'd rather I make those changes :)

[neirbowj]

1. The freebsd stanza should respect link_args and compile_args if they're already set - this allows for proper overriding with the existing env vars.

Unless I'm missing something, it already works this way. More to the point, the FBSD logic only affects kc (whether to rely on the PATH by default, or call by absolute path), which is the later used to learn link_args and compile_args if they are not already supplied in the environment.

2. Optionally, some of the code in setup.py could move around to make more sense, or move into helper functions for clarity. I think classes is probably overkill, though.

Makes complete sense to me. That's a little more than I think I can sign up to at the moment, though.

3. As an optional add-on, we add a variable for the location of krb5_config. This is then used to derive link_args and compile_args if they're not set (and the it makes sense on the platform etc.).

Now I really feel like I'm missing something. kc is a variable with the location of krb5-config. It is already used to set link_args and compile_args if they are not already set, and they are not known in a platform-dependent manner.

[frozencemetery]

Hmm, you're right - I managed to confuse myself somewhere in there. We merely set krb5-config, but don't actually invoke it. So this should be fine then.

[frozencemetery]

@jborean93 Before I merge, are you okay with the new variable name?

[jborean93]

My apologies @frozencemetery just saw this, the new name is fine with me (although it's already merged :)).