Add CRYSTAL_CONFIG_CC compiler config

[straight-shoota]

This environment variable allows to bake into the compiler a default path for CC pointing to a specific linker.
Currently, the fixed default default is cc (cl.exe on Windows). Changing this when compiling the compiler allows taking into account a linker designed for a specific build environment. An example for this would be homebrew, where the compiler should use the linker installed in homebrew because it knows where to find libraries installed via homebrew.

An alternative would be patching the source when necessary, but that's more fragile. An environment variable is more reliable. It serves as a reference that this value can be configured.

Originally suggested in https://forum.crystal-lang.org/t/crystal-installation-using-linuxbrew-is-not-working/6559/10?u=straight-shoota

[crysbot]

This pull request has been mentioned on Crystal Forum. There might be relevant details there:

https://forum.crystal-lang.org/t/crystal-installation-using-linuxbrew-is-not-working/6559/15

[HertzDevil]

I'm wondering about the discoverability of this value. IMO crystal build --verbose shouldn't be the only way to find out the default compiler / linker

[straight-shoota]

It should probably be in the output of crystal env. But then it should be the actual value, taking runtime configuration (i.e. CC) into account.
That's however not trivial due to the complexity of Compiler#linker_command. We should think about how to extract the static linker information.

[straight-shoota]

For stdlib/compiler development, there's a catch: This approach doesn't work when using the makefile because it defines $CC and thus overrides the compiler default.

crystal/Makefile

Line 51 in 2b0b506

EXPORT_CC ?= CC="$(CC) -fuse-ld=lld"