Fix <cmath> compilation on FreeBSD

[dim]

Summary

On FreeBSD, a number of math.h functions are actually defined as macros,
such as signbit(), fpclassify() and others. Since libc++'s <cmath>
attempts to do using ::signbit;, using ::fpclassify;, and so on,
this results in compile errors:

libcxx/include/cmath:309:9: error: '::signbit' has not been declared
 using ::signbit;
         ^
libcxx/include/cmath:310:9: error: '::fpclassify' has not been declared
 using ::fpclassify;
         ^

Here is a patch to exclude signbit through isunordered, and also
abs, which we don't have in math.h.

(Actually, I'm not sure that ::abs should even be in here, since it is
a stdlib.h function? We don't have it in math.h, in any case.)

[dim]

Note that these compilation errors came up specifically because @bdrewery is doing cross-compilation of FreeBSD with recent versions of gcc. Apparently clang does not give the same errors on these undefined identifiers, but I'm not entirely sure why not...

[EricWF]

In D18501#384285, @dim wrote:

Note that these compilation errors came up specifically because @bdrewery is doing cross-compilation of FreeBSD with recent versions of gcc. Apparently clang does not give the same errors on these undefined identifiers, but I'm not entirely sure why not...

Can we find out why?

[rsmith]

libc++'s <math.h> converts the signbit macro into a function. If you're not seeing a function here, that suggests your include paths are messed up and the C library <math.h> is being found before libc++'s <math.h>.

[dim]

Yes, this was indeed the wrong include order. During the latter stages of FreeBSD's buildworld, the compiler is invoked with -nostdinc++, but /usr/include was put before the libc++ include directory, /usr/include/c++/v1. So as @rsmith remarked, the system's math.h was found before the one from libc++, causing the complaints about missing declarations.

include/cmath

 #include <math.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+#ifndef __FreeBSD__
 using ::signbit;
 using ::fpclassify;
 using ::isfinite;
 using ::isinf;
 using ::isnan;
 using ::isnormal;
 using ::isgreater;
 using ::isgreaterequal;
 using ::isless;
 using ::islessequal;
 using ::islessgreater;
 using ::isunordered;
 using ::isunordered;
+#endif // __FreeBSD__
 
 using ::float_t;
 using ::double_t;
 
-#ifndef _AIX
+#if !defined(_AIX) && !defined(__FreeBSD__)
 using ::abs;
-#endif
+#endif // !AIX && !__FreeBSD__
 
 #ifndef __sun__
 using ::acos;
 using ::acosf;
 using ::asin;
 using ::asinf;
 using ::atan;
 using ::atanf;