report that posix_spawn is supported in unistd.h and sysconf

include/unistd.h

@@ -215,6 +215,7 @@ void syncfs(int);
 #define _POSIX_REGEXP           1
 #define _POSIX_SAVED_IDS        1
 #define _POSIX_SHELL            1
+#define _POSIX_SPAWN            _POSIX_VERSION
 #define _POSIX_VDISABLE         0
 
 #define _POSIX_THREADS          _POSIX_VERSION

src/conf/sysconf.c

@@ -171,7 +171,7 @@ long sysconf(int name)
 		[_SC_REGEX_VERSION] = -1,
 		[_SC_SHELL] = 1,
 		[_SC_SIGNALS] = -1,
-		[_SC_SPAWN] = -1,
+		[_SC_SPAWN] = VER,
 		[_SC_SPORADIC_SERVER] = -1,
 		[_SC_THREAD_SPORADIC_SERVER] = -1,
 		[_SC_SYSTEM_DATABASE] = -1,