ExternalData: Warn on missing file instead of failing

When the primary source tree path named by a DATA{} reference does not
exist, produce an AUTHOR_WARNING instead of a FATAL_ERROR.  This is
useful when writing a new DATA{} reference to a test reference output
that has not been created yet.  This way the developer can run the test,
manually verify the output, and then copy it into place to provide the
reference and eliminate the warning.

If the named source tree path is expected to be a file but exists as a
directory, we still need to produce a FATAL_ERROR.

1 files changed, 3 insertions, 2 deletions

diff --git a/Tests/RunCMake/ExternalData/MissingData-stderr.txt b/Tests/RunCMake/ExternalData/MissingData-stderr.txt
index e794f95..39ed2f1 100644
--- a/Tests/RunCMake/ExternalData/MissingData-stderr.txt
+++ b/Tests/RunCMake/ExternalData/MissingData-stderr.txt
@@ -1,4 +1,4 @@
-CMake Error at .*/Modules/ExternalData.cmake:[0-9]+ \(message\):
+CMake Warning \(dev\) at .*/Modules/ExternalData.cmake:[0-9]+ \(message\):
   Data file referenced by argument
 
     DATA{MissingData.txt}
@@ -10,5 +10,6 @@ CMake Error at .*/Modules/ExternalData.cmake:[0-9]+ \(message\):
   that does not exist as a file \(with or without an extension\)!
 Call Stack \(most recent call first\):
   .*
-  MissingData.cmake:2 \(ExternalData_Add_Test\)
+  MissingData.cmake:4 \(ExternalData_Expand_Arguments\)
   CMakeLists.txt:3 \(include\)
+This warning is for project developers.  Use -Wno-dev to suppress it.