GCC 7.3 complains that this is not a compile time constant.

test/format-test.cc: In member function ‘virtual void FormatTest_FormatStringErrors_Test::TestBody()’:
test/format-test.cc:1919:3: error: non-constant condition for static assertion
   static_assert(test_error<__VA_ARGS__>(fmt, error), "")
   ^
test/format-test.cc:1924:3: note: in expansion of macro ‘EXPECT_ERROR’
   EXPECT_ERROR("{0:s", "unknown format specifier", Date);
   ^~~~~~~~~~~~

GCC 7.3 complains about uninitialized varaibles in constexpr context.

C++11 does not support deduction of return type.

Found with GCC and -Wzero-as-null-pointer-constant

OpenBSD doesn't have strtod_l() (at least under the default
headers), so the class Locale that the macro gates won't compile.

Signed-off-by: Andrew Gunnerson <andrewgunnerson@gmail.com>