Support newlib C library configurations without tm_gmtoff field

Newlib C library (https://sourceware.org/newlib/) has a configuration
option to add tm_gmtoff field to the tm structure. Not all the
platforms supported by newlib enable this option, and spdlog doesn't
compile on such platforms due to missing tm_gmtoff field.

Fix this by checking for `__NEWLIB__` and `__TM_GMTOFF` and enabling
calculate_gmt_offset.

include/spdlog/details/os-inl.h

@@ -286,7 +286,7 @@ SPDLOG_INLINE int utc_minutes_offset(const std::tm &tm)
     return offset;
 #else
 
-#    if defined(sun) || defined(__sun) || defined(_AIX) || (!defined(_BSD_SOURCE) && !defined(_GNU_SOURCE))
+#    if defined(sun) || defined(__sun) || defined(_AIX) || (defined(__NEWLIB__) && !defined(__TM_GMTOFF)) || (!defined(_BSD_SOURCE) && !defined(_GNU_SOURCE))
     // 'tm_gmtoff' field is BSD extension and it's missing on SunOS/Solaris
     struct helper
     {