Merge pull request #685 from jwuttke/cmake-idiom1

make CMake module more idiomatic

cmake/generic/modules/PreventInSourceBuilds.cmake

@@ -6,7 +6,7 @@ function(prevent_in_source_builds)
   get_filename_component(bindir "${CMAKE_BINARY_DIR}" REALPATH)
 
   # disallow in-source builds
-  if("${srcdir}" STREQUAL "${bindir}")
+  if(srcdir STREQUAL bindir)
     message(FATAL_ERROR "\
 
 CMake must not to be run in the source directory. \