As with all new Solaris releases, you need to reinstall gcc after upgrading. Unfortunately, Solaris 2.6 include files don't get properly fixed by fixincludes. To fix this, apply the following patch to fixinc.svr4 before building gcc.
*** ./fixinc.svr4.org Thu Jun 15 23:03:29 1995 --- ./fixinc.svr4 Thu Sep 4 13:41:29 1997 *************** *** 189,194 **** --- 189,195 ---- s/__STDC__[ ][ ]*==[ ][ ]*1/defined (__STRICT_ANSI__)/g s/__STDC__[ ][ ]*!=[ ][ ]*0/defined (__STRICT_ANSI__)/g s/__STDC__ - 0 == 0/!defined (__STRICT_ANSI__)/g + s/__STDC__ - 0 == 1/defined (__STRICT_ANSI__)/g /^typedef[ ][ ]*[unsigned ]*long[ ][ ]*[u_]*longlong_t;/s/long/long long/ ' $2/$file > $2/$file.sed mv $2/$file.sed $2/$file