CMake Custom Module Paths

[williamposey]

Path variables for the custom module source files should be added to the cryptolib CMake files. If the variable is not set, then a default path can be used. For example, rather than using aux_source_directory(../../sa/custom SA_CUSTOM_FILES) in CMakeLists.txt it would contain aux_source_directory(SA_CUSTOM_PATH SA_CUSTOM_FILES). Somewhere else, possibly in the top level CMakeLists.txt file, a check could be included to set the path to a default value if not already set:

if(NOT DEFINED SA_CUSTOM_PATH)
  set(SA_CUSTOM_PATH ../../sa/custom)
endif()

This should be done for all custom modules, as it would allow for the open source to be mirrored without any modification more easily