Add options to compile only PG module/frontend

It makes it easier to install Nominatim server and PostgresSQL on
separate servers or use separate docker images for them.

module/CMakeLists.txt

@@ -1,4 +1,13 @@
 # just use the pgxs makefile
+find_program(PG_CONFIG pg_config)
+execute_process(COMMAND ${PG_CONFIG} --pgxs
+            OUTPUT_VARIABLE PGXS
+            OUTPUT_STRIP_TRAILING_WHITESPACE)
+
+if (NOT EXISTS "${PGXS}")
+message(FATAL_ERROR "Postgresql server package not found.")
+endif()
+
 ADD_CUSTOM_COMMAND( OUTPUT ${CMAKE_CURRENT_SOURCE_DIR}/dummy
         COMMAND PGXS=${PGXS} PG_CONFIG=${PG_CONFIG} MODSRCDIR=${CMAKE_CURRENT_SOURCE_DIR} $(MAKE) -f ${CMAKE_CURRENT_SOURCE_DIR}/Makefile
         COMMENT "Running external makefile ${PGXS}"