Workaround a bug in gcc 6 (#4318)

Author: vitaut

include/fmt/base.h

@@ -96,9 +96,9 @@
 // Detect C++14 relaxed constexpr.
 #ifdef FMT_USE_CONSTEXPR
 // Use the provided definition.
-#elif FMT_GCC_VERSION >= 600 && FMT_CPLUSPLUS >= 201402L
-// GCC only allows throw in constexpr since version 6:
-// https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67371.
+#elif FMT_GCC_VERSION >= 702 && FMT_CPLUSPLUS >= 201402L
+// GCC only allows constexpr member functions in non-literal types since 7.2:
+// https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66297.
 #  define FMT_USE_CONSTEXPR 1
 #elif FMT_ICC_VERSION
 #  define FMT_USE_CONSTEXPR 0  // https://github.com/fmtlib/fmt/issues/1628