Use "%option prefix" to set API names in ecpg's lexer.

Use "%option prefix" to set API names in ecpg's lexer.

Back-patch commit 92fb64983 into the pre-9.6 branches.

Without this, ecpg fails to build with the latest version of flex.
It's not unreasonable that people would want to compile our old branches
with recent tools.  Per report from Дилян Палаузов.

Discussion: https://postgr.es/m/d845c1af-e18d-6651-178f-9f08cdf37e10@aegee.org