[Bug libstdc++/60401] stdlib.h does not provide abs(long) overload

[vogt at linux dot vnet.ibm.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60401

--- Comment #6 from Dominik Vogt <vogt at linux dot vnet.ibm.com> ---
The fix seems to be incomplete.  This program still does not compile:

-- snip --
#include <math.h>
static long double (*p3c_)(long double) = acosh;
-- snip --

  $ g++ -std=c++11 -S x.C
  x.C:3:43: error: invalid conversion from ‘double (*)(double) throw ()’ to
‘long double (*)(long double)’ [-fpermissive]
   static long double (*p3c_)(long double) = acosh;
                                             ^~~~~
  $ g++ --version
  g++ (GCC) 6.0.0 20160120 (experimental)