Missing <sys/cdefs.h> on systems with musl libc

[doughdemon]

Could you revert commit 3923ae6d64c4d06fb5b5c21f767c4dda2c304c68 ? <sys/cdefs.h> is not available on all system including those using musl libc. src/main/select.c now already provides a definition of __STRING if not already defined. Alternatively, the configure script could check for <sys/cdefs.h>.