Check for a .gitattributes file in the source dir

Be proactive about making sure a .gitattributes file doesn't introduce
problems into the source tree.  Use the error message to direct devs to
the discussion file with more details.

CMakeLists.txt

@@ -105,6 +105,15 @@ set(BRLCAD_CMAKE_DIR "${BRLCAD_SOURCE_DIR}/misc/CMake")
 list(APPEND CMAKE_MODULE_PATH "${BRLCAD_CMAKE_DIR}")
 
 
+#---------------------------------------------------------------------
+# We definitely do not want a .gitattributes files present, as it is
+# a potential source of subtle problems.  Bail with an explanation if
+# it is found to be present.
+if (EXISTS "${BRLCAD_SOURCE_DIR}/.gitattributes")
+  message(FATAL_ERROR "\nBRL-CAD does not use a .gitattributes file in its repository.  This is intended to prevent subtle errors from creeping in due to inadvertant pattern matches.  See ${BRLCAD_SOURCE_DIR}/doc/git/mime_types.txt for an in-depth discussion of the recommended alternatives to use for the problems .gitattributes is intended to address.\n")
+endif (EXISTS "${BRLCAD_SOURCE_DIR}/.gitattributes")
+
+
 #---------------------------------------------------------------------
 # Setup and checks related to system environment settings.  Some of
 # these impact search results needed to set default options, so we