Configuring Qt for custom Postgres libs via PSQL_LIBS does not work when PSQL_LIBS=xx ist specified in the configure command:
configure.bat [...] -sql-psql -I C:\Path\to\libpq\include -L C:\Path\to\libpq\lib PSQL_LIBS=-llibpqdll

In contrast, It works if PSQL-LIBS is set as environment variable before configure is called:
set PSQL_LIBS=-llibpqdll
configure.bat [...] -sql-psql -I C:\Path\to\libpq\include -L C:\Path\to\libpq\lib