build: Use EGREP to generate graphviz_version.h

build: Use EGREP to generate graphviz_version.h

According to POSIX, '|' only works as an alternation character in grep
when it is extended regular expression mode (egrep, or grep -E).

This is not an issue most of the time because GNU grep accepts it in
non-extended expressions as well, but other grep implementations
adhering to the standard will fail to match the pattern used to generate
graphviz_version.h.

Switching to EGREP (which also requires not escaping the '|'s) makes it
work for all implementations.