Import C++ Standard Template Library

You can now use the hardest fastest and most dangerous language there is
with Cosmopolitan. So far about 75% of LLVM libcxx has been added. A few
breaking changes needed to be made to help this go smoothly.

- Rename nothrow to dontthrow
- Rename nodiscard to dontdiscard
- Add some libm functions, e.g. lgamma, nan, etc.
- Change intmax_t from int128 to int64 like everything else
- Introduce %jjd formatting directive for int128_t
- Introduce strtoi128(), strtou128(), etc.
- Rename bsrmax() to bsr128()

Some of the templates that should be working currently are std::vector,
std::string, std::map, std::set, std::deque, etc.

libc/isystem/features.h

@@ -0,0 +1,38 @@
+#ifndef COSMOPOLITAN_LIBC_ISYSTEM_FEATURES_H_
+#define COSMOPOLITAN_LIBC_ISYSTEM_FEATURES_H_
+
+#if defined(_ALL_SOURCE) && !defined(_GNU_SOURCE)
+#define _GNU_SOURCE 1
+#endif
+
+#if defined(_DEFAULT_SOURCE) && !defined(_BSD_SOURCE)
+#define _BSD_SOURCE 1
+#endif
+
+#if !defined(_POSIX_SOURCE) && !defined(_POSIX_C_SOURCE) && \
+    !defined(_XOPEN_SOURCE) && !defined(_GNU_SOURCE) &&     \
+    !defined(_BSD_SOURCE) && !defined(__STRICT_ANSI__)
+#define _BSD_SOURCE   1
+#define _XOPEN_SOURCE 700
+#endif
+
+#if __STDC_VERSION__ >= 199901L
+#define __restrict restrict
+#elif !defined(__GNUC__)
+#define __restrict
+#endif
+
+#if __STDC_VERSION__ >= 199901L || defined(__cplusplus)
+#define __inline inline
+#elif !defined(__GNUC__)
+#define __inline
+#endif
+
+#if __STDC_VERSION__ >= 201112L
+#elif defined(__GNUC__)
+#define _Noreturn __attribute__((__noreturn__))
+#else
+#define _Noreturn
+#endif
+
+#endif /* COSMOPOLITAN_LIBC_ISYSTEM_FEATURES_H_ */