support musl

[YangKeao]

Related with #27 #32

pprof-rs cannot be compiled and work well with musl 😿 . Find the reason and fix them 😸

[xhebox]

UPDATE: @YangKeao

1. build failure on musl host #27 seems related to protobuf. If the host does not install protobuf, pprof will fallback to the builtin one, which is compiled under glibc(not a valid executable under musl). Either do not fallback if the host is musl, or find a compatible executable for musl. The link to the prost issue, https://github.com/danburkert/prost/issues/202.
2. pthread_setname_np has landed in musl for a long time. And pthread_getname_np is in review. Maybe just directly link to libc to enable the compilation, and ask for the latest musl release. Rust libc wont make the change as it is stuck to a musl version detection problem, they dont even follow up the time_t(32 to 64 on 32bit host) change...

[Xuanwo]

@xhebox Can you help verify this PR #110 too? Thanks in advance.

[xhebox]

@Xuanwo Compilation works, though. Should this issue be closed? #32 is still opened, and pthread_getname_np will be in the next release(1.2.3) of musl.

EDIT: musl 1.2.2 is released 14 months ago.

[xhebox]

For your information, musl has released 1.2.3, i.e. pthread_getname_np is now in the official release, though it will not be present in the standard rust c library.