Merged changes from trunk

* r3890: expr: added support for operator ?: (short ifelse test version)
* r3888: configure: check whether netCDF4/HDF5 is threadsafe
* r3887: vertint: skip vars on surface level
* r3886: wildcard.test: skip wildcard tests
* r3885: set revision number to 1.6.8
* r3884: configure: check whether netCDF4/HDF5 is threadsafe
* r3883: expr: docu update
* r3882: expr: added support for logical operator <=>
* r3881: expr_yacc.c update
* r3880: expr_lex.c update
* r3879: expr_lex.c update
* r3878: expr_lex.c update
* r3877: setmisstoc: added warning if missing value and constant are the same
* r3876: EOF3d: cleanup
* r3875: EOF3D: added function get_weightmode()
* r3874: EOF: added function get_weightmode()
* r3873: makedoc update
* r3872: EOF: added function get_eigenmode()
* r3869: Remap: renamed env. variable  CDO_REMAP_SEARCH_RADIUS to CDO_REMAP_RADIUS
* r3867: Remap: renamed env. variable CDO_REMAP_NORMALIZE_OPT to CDO_REMAP_NORM