Add musl support

[eriweb]

Currently building on alpine fails with:

/src/unit-0.1 # cc -Wl,-E  -o build/unitd -pipe -fPIC -fvisibility=hidden -O -W -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wmissing-prototypes -Werro
r -g   build/src/nxt_main.o  build/libnxt.a -lm -lrt -lpthread
build/libnxt.a(nxt_fiber.o): In function `nxt_fiber_switch_stack':
/src/unit-0.1/src/nxt_fiber.c:214: undefined reference to `getcontext'
/src/unit-0.1/src/nxt_fiber.c:223: undefined reference to `makecontext'
/src/unit-0.1/src/nxt_fiber.c:229: undefined reference to `setcontext'
collect2: error: ld returned 1 exit status

Presumably because getcontext/makecontext/setcontext are deprecated posix api's not implemented by musl.

[VBart]

Could you try this patch?

[eriweb]

Can confirm that it compiles using the latest source and the patch from @VBart

[VBart]

Thank you! Committed as 45c1d41.