Change default CONFIG to none

[KrystalDelusion]

Default to using provided CXX (either via environment variable or make).

Some of the ifeq ($(CONFIG), ..) checks might also be worth changing to check $(CXX) instead to, e.g. use a clang sanitizer if one is selected without config-clang.

[mmicko]

This changes default behavior since now CXX is detected and on Linux it gets defaulted to g++, and current Makefile is enforcing clang by default.

Suggest removing -Wno-c++11-narrowing from Makefile since that will make ABCMKARGS behave same on gcc and clang config paths. (best checking building abc by removing abc directory before build)

[whitequark]

This changes default behavior since now CXX is detected and on Linux it gets defaulted to g++, and current Makefile is enforcing clang by default.

To be honest it was pretty weird that clang would be picked by default even on Linux (I strongly prefer clang but it's still weird).

[KrystalDelusion]

This changes default behavior since now CXX is detected and on Linux it gets defaulted to g++, and current Makefile is enforcing clang by default.

There's nowhere that actually says what the default compiler is, just that running make config-* was necessary for a specific compiler so I don't think that's a problem. And as someone who prefers clang I would always run make config-clang on a fresh install, having assumed until making this PR that gcc would be the default.