The presence of long long int is now implied in the requirement for
C99 and the configure check for the same.

We keep the define hard-coded in ecpg_config.h for backward
compatibility with ecpg-using user code.

Discussion: https://www.postgresql.org/message-id/flat/5cdd6a2b-b2c7-c6f6-344c-a406d5c1a254%402ndquadrant.com