Fix compilation on Android.

include/spdlog/details/os.h

@@ -342,7 +342,9 @@ inline std::string errno_str(int err_num)
     else
         return "Unkown error";
 
-#elif defined(__FreeBSD__) || defined(__APPLE__) || ((_POSIX_C_SOURCE >= 200112L) && ! _GNU_SOURCE) // posix version
+#elif defined(__FreeBSD__) || defined(__APPLE__) || defined(ANDROID) || \
+      ((_POSIX_C_SOURCE >= 200112L) && ! _GNU_SOURCE) // posix version
+
     if (strerror_r(err_num, buf, buf_size) == 0)
         return std::string(buf);
     else
@@ -356,5 +358,3 @@ inline std::string errno_str(int err_num)
 } //os
 } //details
 } //spdlog
-
-