Recently the gcc warning (as errors) settings were changed.
This change triggers compile errors on Linux at some places.
Example :

/OpenJDK/8210319/jdk/src/java.base/unix/native/libnet/net_util_md.h:50:7: error: "__solaris__" is not defined [-Werror=undef]

Here the check
#elif __solaris__
needs to be changed to #elif defined(__solaris__)

There are more platform-checks in native code with same patterns.