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/ipc.h

@@ -4,6 +4,8 @@
 extern "C" {
 #endif
 
+#include <features.h>
+
 #define __NEED_uid_t
 #define __NEED_gid_t
 #define __NEED_mode_t

include/sys/mman.h

@@ -4,6 +4,8 @@
 extern "C" {
 #endif
 
+#include <features.h>
+
 #define __NEED_mode_t
 #define __NEED_size_t
 #define __NEED_off_t

include/sys/resource.h

@@ -5,6 +5,8 @@
 extern "C" {
 #endif
 
+#include <features.h>
+
 #define __NEED_id_t
 #define __NEED_time_t
 #define __NEED_struct_timeval

include/sys/sem.h

@@ -4,6 +4,8 @@
 extern "C" {
 #endif
 
+#include <features.h>
+
 #define __NEED_size_t
 #define __NEED_pid_t
 #define __NEED_time_t

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);

include/sys/shm.h

@@ -5,6 +5,8 @@
 extern "C" {
 #endif
 
+#include <features.h>
+
 #define __NEED_time_t
 #define __NEED_size_t
 #define __NEED_pid_t

include/sys/stat.h

@@ -4,11 +4,7 @@
 extern "C" {
 #endif
 
-#if __STDC_VERSION__ >= 199901L
-#define __restrict restrict
-#elif !defined(__GNUC__)
-#define __restrict
-#endif
+#include <features.h>
 
 #define __NEED_dev_t
 #define __NEED_ino_t

include/sys/statfs.h

@@ -5,6 +5,8 @@
 extern "C" {
 #endif
 
+#include <features.h>
+
 #include <sys/statvfs.h>
 
 typedef struct {

include/sys/statvfs.h

@@ -5,11 +5,7 @@
 extern "C" {
 #endif
 
-#if __STDC_VERSION__ >= 199901L
-#define __restrict restrict
-#elif !defined(__GNUC__)
-#define __restrict
-#endif
+#include <features.h>
 
 #define __NEED_fsblkcnt_t
 #define __NEED_fsfilcnt_t

include/sys/time.h

@@ -4,11 +4,7 @@
 extern "C" {
 #endif
 
-#if __STDC_VERSION__ >= 199901L
-#define __restrict restrict
-#elif !defined(__GNUC__)
-#define __restrict
-#endif
+#include <features.h>
 
 #include <sys/select.h>
 

include/sys/types.h

@@ -4,6 +4,8 @@
 extern "C" {
 #endif
 
+#include <features.h>
+
 #define __NEED_ino_t
 #define __NEED_dev_t
 #define __NEED_uid_t

include/sys/uio.h

@@ -5,6 +5,8 @@
 extern "C" {
 #endif
 
+#include <features.h>
+
 #define __NEED_size_t
 #define __NEED_ssize_t
 #define __NEED_struct_iovec

include/sys/utsname.h

@@ -5,6 +5,8 @@
 extern "C" {
 #endif
 
+#include <features.h>
+
 struct utsname
 {
 	char sysname[65];

include/sys/wait.h

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