Supporting musl libc

[Jacob Carlborg via Digitalmars-d]

As an alternative to glibc there's a C standard library called musl [1]. 
This is the C standard library used by ELLCC [2], a cross-compiler based 
on Clang. This cross-compiler makes it very easy to target other 
platforms and can be used as the C compiler when building with LDC.

The issue is that musl doesn't support the functions defined by 
execinfo.h: backtrace, backtrace_symbols_fd and backtrace_symbols, since 
these are glibc extensions. As far as I can see, these functions are 
used in two places in druntime: src/rt/backtrace/dwarf.d [3] and 
src/core/runtime.d [4].

The imports of execinfo is guarded by version(CRuntime_Glibc). I see 
that CRuntime_Glibc is a predefined version identifier defined by the 
compiler on Linux.

I'm not sure how to best handle different C standard libraries when it 
comes to choosing which one to use. Is it best to choose that when 
building the compiler or when building druntime? Or can it be a runtime 
option?

[1] https://www.musl-libc.org
[2] http://ellcc.org
[3] 
https://github.com/dlang/druntime/blob/master/src/rt/backtrace/dwarf.d#L41
[4] 
https://github.com/dlang/druntime/blob/master/src/core/runtime.d#L433-L434

-- 
/Jacob Carlborg