QGis can be built with or without PostgreSQL/PostGIS support.
Requirements common to either build:
1. Qt 3.0.x built with thread support
   (http://www.trolltech.com/developer/download/qt-x11.html)
2. GDAL/OGR library compiled and installed (with headers). QGIS assumes
   that the headers and library will be in a standard (i.e. /usr/local)
   location. A Linux binary is available at http://www.remotesensing.org/gdal.
   
Additional requirements for building with PostgreSQL/PostGIS support:
1. PostgreSQL built with the --with-CXX configure options (C++ wrapper)
   (http://www.postgresql.org)
2. PostGIS support in PostgreSQL  (http://postgis.refractions.net)

To build QGIS without PostgreSQL/PostGIS support:
1. Make sure your QTDIR environment variable is set to your Qt directory
2. Change to the qgis directory
3. Type: make -f Makefile.nopg

To build QGIS with PostgreSQL/PostGIS support:
2. Set the PGSQL environment variable to point to your PostgreSQL installation
   and make sure the libpq++ headers are available in $(PGSQL)/include
3. Build QGIS by typing: make

The QGIS executable (qgis) will be built in the ./qgis/src directory

For more detailed instructions, see http://qgis.sourceforge.net/qgis_howto.html