boost: fix build on ARC

This commit adds a patch to Boost to make it use the eventfd()
function provided by the C library when uClibc is used, rather than
falling back to using directly the __NR_eventfd system call. This
fixes the build on ARC, which doesn't define __NR_eventfd.

The original problem is that uClibc pretends to be glibc 2.2, which
didn't had eventfd(), so Boost makes the system call
manually. uClibc-ng, in its next release, will pretend to be glibc
2.10 (see
http://cgit.uclibc-ng.org/cgi/cgit/uclibc-ng.git/commit/?id=4ff3a6c8eb91db71d6dc3d2932b66e848bd20ac3),
which will also fix the problem, but requires bumping the uClibc
version, rebuilding the external toolchains, and so on.

Ideally, Boost should be doing a compile test to detect if eventfd()
is available or not, but the Boost build system is so brain-damaged
that doing so would require way too much effort.

Fixes:

  http://autobuild.buildroot.org/results/22b/22b710346d2cd78b7b51cdccd18d670bb6ac5d24/
  and many similar build failures

[Peter: minor tweaks to description]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/boost/0004-fix-uclibc-eventfd.patch

@@ -0,0 +1,38 @@
+Use eventfd() function with uClibc
+
+The Boost eventfd code either directly makes the eventfd system call
+using __NR_eventfd (when __GLIBC_MINOR is less than 8), or otherwise
+uses the eventfd() function provided by the C library.
+
+However, since uClibc pretends to be glibc 2.2, the Boost eventfd code
+directly uses the system call. While it works fine on most
+architectures, it doesn't on ARC since __NR_eventfd is not defined on
+this architecture. However, eventfd() is properly implemented.
+
+So, this patch adjusts the logic used by Boost to consider uClibc as a
+C library providing the eventfd() function.
+
+Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
+
+Index: b/boost/asio/detail/impl/eventfd_select_interrupter.ipp
+===================================================================
+--- a/boost/asio/detail/impl/eventfd_select_interrupter.ipp
++++ b/boost/asio/detail/impl/eventfd_select_interrupter.ipp
+@@ -23,7 +23,7 @@
+ #include <sys/stat.h>
+ #include <sys/types.h>
+ #include <fcntl.h>
+-#if __GLIBC__ == 2 && __GLIBC_MINOR__ < 8
++#if __GLIBC__ == 2 && __GLIBC_MINOR__ < 8 && !defined(__UCLIBC__)
+ # include <asm/unistd.h>
+ #else // __GLIBC__ == 2 && __GLIBC_MINOR__ < 8
+ # include <sys/eventfd.h>
+@@ -46,7 +46,7 @@
+ 
+ void eventfd_select_interrupter::open_descriptors()
+ {
+-#if __GLIBC__ == 2 && __GLIBC_MINOR__ < 8
++#if __GLIBC__ == 2 && __GLIBC_MINOR__ < 8 && !defined(__UCLIBC__)
+   write_descriptor_ = read_descriptor_ = syscall(__NR_eventfd, 0);
+   if (read_descriptor_ != -1)
+   {