Check BUILD_LEGACY_64_BIT_MODE instead of BUILD_REAL_64_BIT_MODE.

Check BUILD_LEGACY_64_BIT_MODE instead of BUILD_REAL_64_BIT_MODE.

Since unixODBC version 2.2.13, the #define that controls whether SQLLEN
and SQLULEN are 32-bits or 64-bits wide is called BUILD_LEGACY_64_BIT_MODE,
not BUILD_REAL_64_BIT_MODE (the default changed to 64-bits in 2.2.13).
2.2.13 was released in 2006, so it's probably safe to assume that psqlodbc
is being built against that or higher.