remove out-of-line defns of constexpr static data members under c++17

Address warnings of this form under c++17 when building with `-Werror -Wdeprecated`:
```
fast_float/float_common.h:446:58: error: out-of-line definition of constexpr static data member is redundant in C++17 and is deprecated [-Werror,-Wdeprecated]
constexpr double binary_format_lookup_tables<double, U>::powers_of_ten[];
                                                         ^
```

include/fast_float/bigint.h

@@ -404,12 +404,16 @@ template <typename = void> struct pow5_tables {
 #endif
 };
 
+#if FASTFLOAT_DETAIL_MUST_DEFINE_CONSTEXPR_VARIABLE
+
 template <typename T> constexpr uint32_t pow5_tables<T>::large_step;
 
 template <typename T> constexpr uint64_t pow5_tables<T>::small_power_of_5[];
 
 template <typename T> constexpr limb pow5_tables<T>::large_power_of_5[];
 
+#endif
+
 // big integer type. implements a small subset of big integer
 // arithmetic, using simple algorithms since asymptotically
 // faster algorithms are slower for a small number of limbs.

include/fast_float/constexpr_feature_detect.h

@@ -37,4 +37,10 @@
 #define FASTFLOAT_IS_CONSTEXPR 0
 #endif
 
+#if __cplusplus >= 201703L || (defined(_MSVC_LANG) && _MSVC_LANG >= 201703L)
+#define FASTFLOAT_DETAIL_MUST_DEFINE_CONSTEXPR_VARIABLE 0
+#else
+#define FASTFLOAT_DETAIL_MUST_DEFINE_CONSTEXPR_VARIABLE 1
+#endif
+
 #endif // FASTFLOAT_CONSTEXPR_FEATURE_DETECT_H

include/fast_float/fast_table.h

@@ -693,10 +693,14 @@ template <class unused = void> struct powers_template {
   };
 };
 
+#if FASTFLOAT_DETAIL_MUST_DEFINE_CONSTEXPR_VARIABLE
+
 template <class unused>
 constexpr uint64_t
     powers_template<unused>::power_of_five_128[number_of_entries];
 
+#endif
+
 using powers = powers_template<>;
 
 } // namespace fast_float

include/fast_float/float_common.h

@@ -442,12 +442,16 @@ template <typename U> struct binary_format_lookup_tables<double, U> {
                           constant_55555 * 5 * 5 * 5 * 5)};
 };
 
+#if FASTFLOAT_DETAIL_MUST_DEFINE_CONSTEXPR_VARIABLE
+
 template <typename U>
 constexpr double binary_format_lookup_tables<double, U>::powers_of_ten[];
 
 template <typename U>
 constexpr uint64_t binary_format_lookup_tables<double, U>::max_mantissa[];
 
+#endif
+
 template <typename U> struct binary_format_lookup_tables<float, U> {
   static constexpr float powers_of_ten[] = {1e0f, 1e1f, 1e2f, 1e3f, 1e4f, 1e5f,
                                             1e6f, 1e7f, 1e8f, 1e9f, 1e10f};
@@ -469,12 +473,16 @@ template <typename U> struct binary_format_lookup_tables<float, U> {
       0x1000000 / (constant_55555 * constant_55555 * 5)};
 };
 
+#if FASTFLOAT_DETAIL_MUST_DEFINE_CONSTEXPR_VARIABLE
+
 template <typename U>
 constexpr float binary_format_lookup_tables<float, U>::powers_of_ten[];
 
 template <typename U>
 constexpr uint64_t binary_format_lookup_tables<float, U>::max_mantissa[];
 
+#endif
+
 template <>
 inline constexpr int binary_format<double>::min_exponent_fast_path() {
 #if (FLT_EVAL_METHOD != 1) && (FLT_EVAL_METHOD != 0)
@@ -677,8 +685,12 @@ template <typename = void> struct space_lut {
       0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0};
 };
 
+#if FASTFLOAT_DETAIL_MUST_DEFINE_CONSTEXPR_VARIABLE
+
 template <typename T> constexpr bool space_lut<T>::value[];
 
+#endif
+
 inline constexpr bool is_space(uint8_t c) { return space_lut<>::value[c]; }
 #endif
 
@@ -759,12 +771,16 @@ template <typename = void> struct int_luts {
       3379220508056640625,     4738381338321616896};
 };
 
+#if FASTFLOAT_DETAIL_MUST_DEFINE_CONSTEXPR_VARIABLE
+
 template <typename T> constexpr uint8_t int_luts<T>::chdigit[];
 
 template <typename T> constexpr size_t int_luts<T>::maxdigits_u64[];
 
 template <typename T> constexpr uint64_t int_luts<T>::min_safe_u64[];
 
+#endif
+
 template <typename UC>
 fastfloat_really_inline constexpr uint8_t ch_to_digit(UC c) {
   return int_luts<>::chdigit[static_cast<unsigned char>(c)];