Disable warning treated as error for signed/unsigned comparison in building splashscreen

The updated giflib seems to suffer from signed/unsigned comparisons which collide
with the newly enabled JDK build default to treat warnings as errors unless explicitly excluded.

On Linux and Windows we are getting a build failure as a result
client/jdk/src/java.desktop/share/native/libsplashscreen/giflib/dgif_lib.c:1074:13: error: signed and unsigned type in conditional expression [-Werror=sign-compare]

c:/jprt/T/P1/171125.prrace/s/jdk/src/java.desktop/share/native/libsplashscreen/giflib/dgif_lib.c(404) : error C2220: warning treated as error - no 'object' file generated
c:/jprt/T/P1/171125.prrace/s/jdk/src/java.desktop/share/native/libsplashscreen/giflib/dgif_lib.c(404) : warning C4018: '<' : signed/unsigned mismatch