default features: make musl usable without feature test macros

the old behavior of exposing nothing except plain ISO C can be
obtained by defining __STRICT_ANSI__ or using a compiler option (such
as -std=c99) that predefines it. the new default featureset is POSIX
with XSI plus _BSD_SOURCE. any explicit feature test macros will
inhibit the default.

installation docs have also been updated to reflect this change.

include/sys/sendfile.h

@@ -5,6 +5,7 @@
 extern "C" {
 #endif
 
+#include <features.h>
 #include <unistd.h>
 
 ssize_t sendfile(int, int, off_t *, size_t);