Improve FindPostgreSQL.cmake

[tbussmann]

Fixes #1157.

Changes proposed in this pull request:

• Prefer the path given by pg_config when searching for POSTGRESQL_EXECUTABLE by changing the search order to avoid matching an unrelated executable

@pgRouting/admins

This PR is the minimal required change to the logic to fix building on Mac in several typical constellations. I would however rather prefer the following bigger change:

• remove all the 'guesses' of fixed-paths and environment variables
• exclusively rely on the output of pg_config, find that according to PATH, overridable by POSTGRESQL_BIN

Reason:

• we already are depending on a working pg_config, otherwise POSTGRESQL_LIBRARIES and POSTGRESQL_VERSION_STRING could not be set, and POSTGRESQL_FOUND would result FALSE
• with the individual searching / guessing logic it is possible to find POSTGRESQL_INCLUDE_DIR POSTGRESQL_LIBRARIESand POSTGRESQL_EXECUTABLE belonging to different installations which cannot work
• it is already possible to specify -DPOSTGRESQL_BIN=... to point to a particular installation not in path of which pg_config would then control the other directories

Con:

• no longer possible to magically build against a installation with a broken pg_config - this would now require to explicitly specify all of POSTGRESQL_INCLUDE_DIR, POSTGRESQL_LIBRARIES, POSTGRESQL_EXECUTABLE, POSTGRESQL_VERSION_STRING as cmake parameters

If you agree, I'll update the PR to reflect the wider change.