exclude pdfwriter testing from git archives (#244)

* .gitattributes file ignoring PDFWriterTesting

* other way around

* condition cmake to check for existance of PDFWriterTesting to add subdirectory, so itll work well in archives, now that there's no PDFWriterTesting there

.gitattributes

@@ -0,0 +1,2 @@
+# exclude tests from exported git archives
+PDFWriterTesting export-ignore

CMakeLists.txt

@@ -153,13 +153,13 @@ endif(NOT USE_BUNDLED)
 
 ADD_SUBDIRECTORY(PDFWriter)
 
-if(PROJECT_IS_TOP_LEVEL)
+if(PROJECT_IS_TOP_LEVEL AND EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/PDFWriterTesting)
     # avoid installing the testing lib altogether when included in another project.
     # it's annoying when in parent all, and more annoying to then get the tests added
     # to the parent project ctest.
     enable_testing()
     ADD_SUBDIRECTORY(PDFWriterTesting)
-endif(PROJECT_IS_TOP_LEVEL)
+endif()
 
 include(InstallRequiredSystemLibraries)
 set(CPACK_RESOURCE_FILE_LICENSE "${CMAKE_CURRENT_SOURCE_DIR}/LICENSE")