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/ucontext.h

@@ -4,6 +4,8 @@
 extern "C" {
 #endif
 
+#include <features.h>
+
 #include <signal.h>
 
 #ifdef _GNU_SOURCE