HPCC-12670 Allow user provide non-default path for required development ...

Allow HPCC build search both user provided libraries path and native path

Previously HPCC only allow user use either "EXTERNALS_DIRECTORY"  or native libraries (IF USE_NATIVE_LIBRARIES is not set to OFF native libraries will overwrite ""EXTERNALS_DIRECTORY" settings) .  With this change user can use both. "EXTERNALS_DIRECTORY" will be searched first if provided. If libraries file or header files not found and "USE_NATIVE_LIBRARIES" is  "ON".  navie libraries path will be tried.

@mckellyln @Michael-Gardner please help review.