Trac #27263: Upgrade to giac 1.5 fails to build with older libstdc++

#26315 broke building giac on some systems that have a old-ish
libstdc++.  It is completely mysterious to me ''why'' this broke when it
worked before, and without a repository it's very difficult to determine
what change between the versions is responsible, as there is little
difference between the two versions in the relevant code.

Nevertheless, it can be fixed by explicitly using `std::isnan` and
`std::isinf` so that there is no risk of them conflicting with the libc
math.h equivalents thereof.

URL: https://trac.sagemath.org/27263
Reported by: embray
Ticket author(s): Erik Bray
Reviewer(s): Dima Pasechnik

build/pkgs/giac/patches/isnan-conflict.patch

@@ -0,0 +1,30 @@
+Just always use std::isnan and std::isinf so there is no risk of
+conflicting with the libc math.h equivalents thereof.
+
+See https://trac.sagemath.org/ticket/27263
+--- a/src/global.cc	2019-02-12 15:49:03.082594000 +0000
++++ b/src/global.cc	2019-02-12 15:49:43.438594000 +0000
+@@ -4139,11 +4139,7 @@
+     return isnan(d);
+ #endif
+ #else
+-#if defined(FIR_LINUX) || defined(FIR_ANDROID)
+     return std::isnan(d);
+-#else
+-    return isnan(d);
+-#endif
+ #endif
+   }
+
+@@ -4152,11 +4148,7 @@
+     double x=0.0;
+     return d==1.0/x || d==-1.0/x;
+ #else
+-#if defined(FIR_LINUX) || defined(FIR_ANDROID)
+     return std::isinf(d);
+-#else
+-    return isinf(d);
+-#endif
+ #endif
+   }
+