[Patch] Fix canadian cross build on systems with no fenv.h

[Steve Ellcey]

I ran into a build problem while doing a canadian cross build of GCC.
I was building on linux to create a Windows (mingw) GCC that generates
code for mips-mti-elf.

The mips-mti-elf target uses newlib for its system headers and libraries
and the headers do not include a fenv.h header file.  However, when doing
a canadian cross build libstdc++ is built with the C++ compiler that runs
on linux (the build system), not the C++ that was just built for Windows
(the host system).  The problem is that the libstdc++ configure script
(in GLIBCXX_CHECK_C99_TR1) is checking for the fenv.h using C++ (not C)
and the C++ compiler running on linux does have an fenv.h header because
the latest libstdc++ builds always create this header for C++ regardless
of whether there is one on the system or not.  This results in the newly
created libstdc++ library having a fenv.h header that tries to include the
system fenv.h header file which does not exist and the build fails.

My fix for this is to explicitly check for fenv.h and complex.h in the
libstdc++ configure.ac script before calling GLIBCXX_CHECK_C99_TR1.
This makes the configure script check for these headers using the C
compiler instead of the C++ compiler and when GLIBCXX_CHECK_C99_TR1
is run it uses that information (saved in a autoconf variable) to 
correctly ascertain that fenv.h does not exist.

I could put these checks in GLIBCXX_CHECK_C99_TR1 if that were considered
preferable, it would just have to be done before we call 'AC_LANG_CPLUSPLUS'.

Tested with both my canadian cross build and a standard cross build
targetting mips-mti-elf.

OK for checkin?

Steve Ellcey
sellcey@mips.com


2013-10-30  Steve Ellcey  <sellcey@mips.com>

	* configure.ac: Add header checks for fenv.h and complex.h.
	* configure: Regenerate.


diff --git a/libstdc++-v3/configure.ac b/libstdc++-v3/configure.ac
index dd13b01..22fc840 100644
--- a/libstdc++-v3/configure.ac
+++ b/libstdc++-v3/configure.ac
@@ -195,6 +195,12 @@ GLIBCXX_CHECK_S_ISREG_OR_S_IFREG
 AC_CHECK_HEADERS(sys/uio.h)
 GLIBCXX_CHECK_WRITEV
 
+# Check for fenv.h and complex.h before GLIBCXX_CHECK_C99_TR1
+# so that the check is done with the C compiler (not C++).
+# Checking with C++ can break a canadian cross build if either
+# file does not exist in C but does in C++.
+AC_CHECK_HEADERS(fenv.h complex.h)
+
 # For C99 support to TR1.
 GLIBCXX_CHECK_C99_TR1