Disable fenv.h in certain configurations

The boost build system does not properly test whether fenv.h is
available, and if it is, if it supports all the features used by
Boost. This causes build failures with uClibc (reported upstream at
https://svn.boost.org/trac/boost/ticket/11756) but also with glibc on
specific architectures that don't have a full fenv implementation,
such as NIOSII or Microblaze.

To address this, we forcefully disable the use of fenv support in the
affected configurations.

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
[Thomas: add Microblaze/NIOSII exclusions.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Index: b/boost/config/platform/linux.hpp
===================================================================
--- a/boost/config/platform/linux.hpp
+++ b/boost/config/platform/linux.hpp
@@ -47,6 +47,16 @@
 #endif
 
 //
+// uClibc has no support for fenv.h, and also a few architectures
+// don't have fenv.h support at all (or incomplete support) even with
+// glibc.
+
+//
+#if defined(__UCLIBC__) || defined(__nios2__) || defined(__microblaze__)
+#  define BOOST_NO_FENV_H
+#endif
+
+//
 // If glibc is past version 2 then we definitely have
 // gettimeofday, earlier versions may or may not have it:
 //
