The unofficial musl manual

Feature test macros, standards:
Musl supports the following standards, with the given feature test macros or parameters:
X/Open 2008 (SUS4)	_XOPEN_SOURCE   >= 700
POSIX 2008		_POSIX_C_SOURCE >= 200809L, _POSIX_SOURCE
ISO C99			(requires -std=c99 or equivalent; detected using __STRICT_ANSI__, which gcc sets automatically)

The following standards are partly supported:
ISO C11			(WIP, threads and atomics aren't supported yet, and 
			Annex K is unlikely to be supported in the foreseeable 
			future; use -std=c11 or equivalent)
ANSI C89/ISO C90	A few details are incompatible with C99, and the 
			namespace respected is the C99 one.  However, there 
			shouldn't be breakage.
Older POSIX and SUS	_POSIX_C_SOURCE < 200809L, _XOPEN_SOURCE < 700
			Exposes legacy extensions as well as the modern 
			standard, rather than only the legacy standard.

The following feature test macros are also supported to the extent practical:
_GNU_SOURCE		Partial glibc compatability.
			Nonstandard functions with the same names as standard 
			ones (basename and several others) ARE NOT SUPPORTED:
			musl uses the ISO/POSIX/SUS definition unconditionally.
_LARGEFILE64_SOURCE	nop: musl only provides 64bit file io, so this just adds
			*64 aliases to the namespace
_BSD_SOURCE		While glibc is still stuck at BSD 4.4, musl provides
			much of the functionality available via _NETBSD_SOURCE
			on NetBSD. This includes strlcpy, strlcat, and fgetln.

By default (-std=gnu* or no arguments for GCC), musl currently sets 
_XOPEN_SOURCE to 700 and defines _BSD_SOURCE.
With musl 0.9.4 or older, musl defaulted to ANSI C99.

Detecting musl:
This is frequently asked about. It is not possible to detect musl for sure.
Key differences:
__linux and __ELF will both be defined, unlike with dietlibc.
unistd.h defines _XOPEN_VERSION to 700 and _POSIX_VERSION to 200809L. 
(This is what you should be checking for, unless you need nonstandard functions)
features.h defaults to defining _BSD_SOURCE and _XOPEN_SOURCE, rather than _BSD_SOURCE and _SVID_SOURCE; __USE_* macros are not present, nor are the __GLIBC* macros.
Using GNU basename syntax ( basename("/usr"); ) will make for segfaults.