Cross compile directions:

Create /usr/share/qt4/mkspecs/win32-cross/qmake.conf
Enter:

# qmake configuration for win32-g++
#
# Written for MinGW
#

MAKEFILE_GENERATOR      = MINGW
TEMPLATE                = app
CONFIG                  += qt warn_on release link_prl copy_dir_files precompile_header
QT                      += core gui
DEFINES                 += UNICODE QT_LARGEFILE_SUPPORT
QMAKE_COMPILER_DEFINES  += __GNUC__ WIN32

QMAKE_EXT_OBJ           = .obj
QMAKE_EXT_RES           = _res.obj

QMAKE_CC                = i586-mingw32-gcc
QMAKE_LEX               = flex
QMAKE_LEXFLAGS          =
QMAKE_YACC              = byacc
QMAKE_YACCFLAGS         = -d
QMAKE_CFLAGS            =
QMAKE_CFLAGS_DEPS       = -M
QMAKE_CFLAGS_WARN_ON    = -Wall
QMAKE_CFLAGS_WARN_OFF   = -w
QMAKE_CFLAGS_RELEASE    = -O2
QMAKE_CFLAGS_DEBUG      = -g
QMAKE_CFLAGS_YACC       = -Wno-unused -Wno-parentheses
QMAKE_CFLAGS_THREAD     = -mthreads

QMAKE_CXX               = i586-mingw32-g++
QMAKE_CXXFLAGS          = $$QMAKE_CFLAGS
QMAKE_CXXFLAGS_DEPS     = $$QMAKE_CFLAGS_DEPS
QMAKE_CXXFLAGS_WARN_ON  = $$QMAKE_CFLAGS_WARN_ON
QMAKE_CXXFLAGS_WARN_OFF = $$QMAKE_CFLAGS_WARN_OFF
QMAKE_CXXFLAGS_RELEASE  = $$QMAKE_CFLAGS_RELEASE
QMAKE_CXXFLAGS_DEBUG    = $$QMAKE_CFLAGS_DEBUG
QMAKE_CXXFLAGS_YACC     = $$QMAKE_CFLAGS_YACC
QMAKE_CXXFLAGS_THREAD   = $$QMAKE_CFLAGS_THREAD
QMAKE_CXXFLAGS_RTTI_ON  = -frtti
QMAKE_CXXFLAGS_RTTI_OFF = -fno-rtti
QMAKE_CXXFLAGS_EXCEPTIONS_ON = -fexceptions
QMAKE_CXXFLAGS_EXCEPTIONS_OFF = -fno-exceptions

QMAKE_INCDIR            =
QMAKE_INCDIR_QT         = /mnt/C/Qt/4.2.3/include/
QMAKE_LIBDIR_QT         = /mnt/C/Qt/4.2.3/lib/

QMAKE_RUN_CC            = $(CC) -c $(CFLAGS) $(INCPATH) -o $obj $src
QMAKE_RUN_CC_IMP        = $(CC) -c $(CFLAGS) $(INCPATH) -o $@ $<
QMAKE_RUN_CXX           = $(CXX) -c $(CXXFLAGS) $(INCPATH) -o $obj $src
QMAKE_RUN_CXX_IMP       = $(CXX) -c $(CXXFLAGS) $(INCPATH) -o $@ $<

QMAKE_LINK              = i586-mingw32-g++
QMAKE_LFLAGS            = -mthreads -Wl,-enable-stdcall-fixup -Wl,-enable-auto-import -Wl,-enable-runtime-pseudo-reloc
QMAKE_LFLAGS_RELEASE    = -Wl,-s
QMAKE_LFLAGS_DEBUG      =
QMAKE_LFLAGS_CONSOLE    = -Wl,-subsystem,console
QMAKE_LFLAGS_WINDOWS    = -Wl,-subsystem,windows
QMAKE_LFLAGS_DLL        = -shared
QMAKE_LINK_OBJECT_MAX   = 10
QMAKE_LINK_OBJECT_SCRIPT= object_script


QMAKE_LIBS              =
QMAKE_LIBS_CORE         = -lkernel32 -luser32 -lshell32 -luuid -lole32 -ladvapi32 -lws2_32
QMAKE_LIBS_GUI          = -lgdi32 -lcomdlg32 -loleaut32 -limm32 -lwinmm -lwinspool -lws2_32 -lole32 -luuid -luser32 -ladvapi32
QMAKE_LIBS_NETWORK      = -lws2_32
QMAKE_LIBS_OPENGL       = -lopengl32 -lglu32 -lgdi32 -luser32
QMAKE_LIBS_COMPAT       = -ladvapi32 -lshell32 -lcomdlg32 -luser32 -lgdi32 -lws2_32
QMAKE_LIBS_QT_ENTRY     = -lmingw32 -lqtmain

MINGW_IN_SHELL = $$(MINGW_IN_SHELL)
QMAKE_DIR_SEP           = /
QMAKE_COPY              = cp
QMAKE_COPY_DIR          = cp -r
QMAKE_MOVE              = mv
QMAKE_DEL_FILE          = rm -f
QMAKE_MKDIR             = mkdir
QMAKE_DEL_DIR           = rm -rf

QMAKE_MOC               = $$[QT_INSTALL_BINS]$${DIR_SEPARATOR}moc
QMAKE_UIC               = $$[QT_INSTALL_BINS]$${DIR_SEPARATOR}uic
QMAKE_IDC               = $$[QT_INSTALL_BINS]$${DIR_SEPARATOR}idc
QMAKE_RCC               = $$[QT_INSTALL_BINS]$${DIR_SEPARATOR}rcc

QMAKE_IDL               = midl
QMAKE_LIB               = i586-mingw32-ar -ru
QMAKE_RC                = i586-mingw32-windres
QMAKE_ZIP               = zip -r -9

QMAKE_STRIP             = i586-mingw32-strip
QMAKE_STRIPFLAGS_LIB    += --strip-unneeded
QMAKE_CHK_DIR_EXISTS    = if not exist

MAKEFILE                = Makefile.win32

load(qt_config)

### End ###

Run:
qmake -win32 -spec win32-cross
make -f Makefile.win32
