Add exclusions file that controls published sources

Add lines  to the sources.exclude  file to select paths  and wildcards
that   will  be   excluded   from  source   tarballs.   See  the   tar
--exclude-from=FILE option for syntax.

If a source  directory that is used to build  targets is included then
those targets should  be defined in a CMakeLists.txt file  in the same
directory  and the  following  syntax  should be  used  in the  parent
CMakeLists.txt script to ensue  the parent CMakeLists.txt file remains
valid when the excluded directory is not present:

if (EXITS ${CMAKE_SOURCE_DIR}/optional-source-dir AND IS_DIRECTORY ${CMAKE_SOURCE_DIR}/optional-source-dir)
   add_subdirectory (optional-source-dir)
endif ()

CMakeLists.txt

@@ -887,7 +887,9 @@ endif (WIN32)
 #
 # sub-directories
 #
-add_subdirectory (samples)
+if (EXISTS ${CMAKE_SOURCE_DIR}/samples AND IS_DIRECTORY ${CMAKE_SOURCE_DIR}/samples)
+  add_subdirectory (samples)
+endif ()
 if (WSJT_GENERATE_DOCS)
   add_subdirectory (doc)
 endif (WSJT_GENERATE_DOCS)

sources.exclude

@@ -0,0 +1,3 @@
+.git
+sources.exclude
+samples