[flang] Accomodate missing clock_gettime(); remove f18-parse-demo dependence...

[flang] Accomodate missing clock_gettime(); remove f18-parse-demo dependence on lib/evaluate; allow #ifdef with no name

Original-commit: flang-compiler/f18@330fd8116febab2416afeb8c08d6216c1edd9e55
Reviewed-on: https://github.com/flang-compiler/f18/pull/335
Tree-same-pre-rewrite: false