Fix build on older Motif without XmPIXMAP_AND_STRING.

Fix build on older Motif without XmPIXMAP_AND_STRING.

It seems that not all versions of Motif support labels with a labelType
of XmPIXMAP_AND_STRING.  This mode is used in the about dialog to
display the program's icon, but it's silly to fail compilation over
this.

Add a configure test to check if this feature is available, and fall
back to XmSTRING (which will render without the icon).