Please replace this commit with a better solution, if you have one.

e1ac5933 brought us the VRBT node
color encoded in the lower two bits of pointers.

gcc strict aliasing rules (as enabled with -O2 and higher) forbid
aliased lvalue access as in the following macro expanded code:

(*(uintptr_t *)&(parent)->entry.rbe_parent) &= ~((uintptr_t)3);

Until we have a better solution, disable strict aliasing with gcc
for targets with any source file using vtree.h.

Note that we would want to limit -fno-strict-aliasing to individual
compliation units, but automake does not offer a simple and clean way
to achieve this:

https://www.gnu.org/software/automake/manual/html_node/Per_002dObject-Flags.html