Initial import of libdpf.

Co-authored-by: Cursor <cursoragent@cursor.com>

thirdparty/portable-snippets/clock/README.md

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+# Clock
+
+This header provides a way to access time information.  It's basically
+a more portable version of `clock_gettime` with fewer supported
+clocks.  Currently supported clocks are:
+
+ * Wall clock
+   * `clock_gettime`
+   * `gettimeofday`
+   * `time`
+ * CPU clock
+   * `clock_gettime`
+   * `GetProcessTimes`
+   * `getrusage`
+ * Monotonic clock
+   * `clock_gettime`
+   * `mach_absolute_time`
+   * `QueryPerformanceCounter`
+
+If you are using a platform where a clock isn't provided, please let
+us know about it so we can try to figure out how to add support!
+
+On some platforms, `clock_gettime` requires linking to librt.  If you
+prefer, you can define `PSNIP_CLOCK_NO_LIBRT` prior to including
+`clock.h` and `clock_gettime` will only be used on platforms known to
+support using `clock_gettime` *without* linking against librt.  The
+price will likely be lower-resolution CPU and wall clocks, and no
+monotonic clock.
+
+Also, if you're compiling in a strict standard mode instead of a mode
+with GNU extensions (e.g., -std=c11 instead of -std=gnu11), POSIX
+extensions will not be enabled by default so you won't get to use the
+`clock_gettime` implementation; to avoid this, you can define
+`_POSIX_C_SOURCE` to `199309L` or greater prior to including
+`clock.h`, or just define `_GNU_SOURCE`.
+
+## Dependencies
+
+To maximize portability you should #include the exact-int module
+before including clock.h, but if you don't want to add the extra
+file to your project you can omit it and this module will simply rely
+on <stdint.h>.  As an alternative you may define `psnip_uint64_t`,
+`psnip_uint32_t`, `psnip_int64_t`, `psnip_int32_t` to an appropriate
+value yourself before including clock.h.