gyp: do not declare POSIX_C_SOURCE on freebsd

On FreeBSD sys/cdefs.h contains following lines:

    #if defined(_POSIX_SOURCE) && !defined(_POSIX_C_SOURCE)
    #define	_POSIX_C_SOURCE		198808
    #endif
    #ifdef _POSIX_C_SOURCE
      /* omitted */
    #else

    #if defined(_ANSI_SOURCE)	/* Hide almost everything. */
    #define	__POSIX_VISIBLE		0
    #define	__XSI_VISIBLE		0
    #define	__BSD_VISIBLE		0
    #define	__ISO_C_VISIBLE		1990
    #elif defined(_C99_SOURCE)	/* Localism to specify strict C99 env. */
    #define	__POSIX_VISIBLE		0
    #define	__XSI_VISIBLE		0
    #define	__BSD_VISIBLE		0
    #define	__ISO_C_VISIBLE		1999
    #else				/* Default environment: show everything. */
    #define	__POSIX_VISIBLE		200809
    #define	__XSI_VISIBLE		700
    #define	__BSD_VISIBLE		1
    #define	__ISO_C_VISIBLE		1999
    #endif
    #endif

If __BSD_VISIBLE is 0, setgroups/getgroups/uchar and etc won't be
available for external users of libuv (like node.js).

/cc @bnoordhuis