#479 libsbml doesn't work as a cmake sub-project

[Andy Somogyi]

Easy fix, in CMakeLists.txt just

Replace:
set(LIBSBML_ROOT_SOURCE_DIR "${CMAKE_SOURCE_DIR}" CACHE PATH
"Path to the libSBML root source directory")

With:
set(LIBSBML_ROOT_SOURCE_DIR "${CMAKE_CURRENT_SOURCE_DIR}" CACHE PATH
"Path to the libSBML root source directory")