message: Fix a bug in tests/unit/CMakeLists.txt that prevented building of
  the unit tests with CMake.
diff: # terms of the GNU Lesser General Public License (as published by the Free
  # Software Foundation) version 2.1 dated February 1999.
  -# Include the build directory where mfem.hpp and mfem-performance.hpp are.
  -include_directories(BEFORE ${PROJECT_BINARY_DIR})
  # Include the top mfem source directory - needed by some tests, e.g. to
  # #include "general/text.hpp".
  include_directories(BEFORE ${PROJECT_SOURCE_DIR})
  +# Include the build directory where mfem.hpp and mfem-performance.hpp are.
  +include_directories(BEFORE ${PROJECT_BINARY_DIR})
  # Include the source directory for the unit tests - catch.hpp is there.
  include_directories(BEFORE ${CMAKE_CURRENT_SOURCE_DIR})
  