Improve inf/nan. This also makes GCC encode them consistently with Clang and…

Improve inf/nan.  This also makes GCC encode them consistently with Clang and with the existing testdata.

c++/src/kj/common.h

@@ -429,8 +429,8 @@ static constexpr MinValue_ minValue = MinValue_();
 //
 // `char` is not supported, but `signed char` and `unsigned char` are.
 
-inline float inf() { return 1.0f / 0.0f; }
-inline float nan() { return 0.0f / 0.0f; }
+inline constexpr float inf() { return __builtin_huge_valf(); }
+inline constexpr float nan() { return __builtin_nanf(""); }
 
 // =======================================================================================
 // Useful fake containers

mega-test-quick.cfg

 linux-gcc-4.7      1704 ./super-test.sh tmpdir capnp-gcc-4.7 quick
 linux-gcc-4.8      1707 ./super-test.sh tmpdir capnp-gcc-4.8 quick gcc-4.8
 linux-clang        1745 ./super-test.sh tmpdir capnp-clang quick clang
-mac                 791 ./super-test.sh remote beat caffeinate quick
+mac                 802 ./super-test.sh remote beat caffeinate quick
 cygwin              795 ./super-test.sh remote Kenton@flashman quick