If postgres was compiled without flex support - possible when building
from a tarball, because those contain the flex generated files - citus
compilation would fail, because FLEX is defined as missing.
Add detection for flex, overwriting postgres' detection if one was
found.
Fixes: #439
All citusdb references in
- extension, binary names
- file headers
- all configuration name prefixes
- error/warning messages
- some functions names
- regression tests
are changed to be citus.