Require bison & flex in configure.

[dzwarg]

If a user doesn't have bison or flex installed when performing ./autogen.sh and ./configure, postgis appears to be configured properly, but running make will complain about the symbol "wktyylex" missing.

The simple fix would be to have configure require bison and flex, and not permit the make process to continue.

[pramsey]

The .c files generated by bison and flex should be committed into the repo, so the lexer/parser shouldn't be required for a build, generally speaking. I will look more closely at my build and see what happens there.

[pramsey]

Oh, and as a general rule, unfortunately bison/flex has enough functionality drift across versions that people with different versions will get regression failures. So we do want to avoid having builds in general generate the .c files from the .y and .l files.

[pramsey]

Not seeing this on my current checkout or on a fresh one. The bison and flex components are not called, because the .c files are checked into the repo. Re-open if this continues.