Add check for presence of realpath to CMakeLists.txt to support more platforms (#7603)
Add check for presence of realpath to CMakeLists.txt to support more platforms (#7603)

* add automatic check for realpath in CMakeLists

* added win32 check

Co-authored-by: Derek Bailey <fba4b034b859aae28ee22dc46404a75392e00155@google.com>
google/flatbuffers,google/flatbuffers,google/flatbuffers,alexames/flatbuffers,google/flatbuffers,google/flatbuffers,google/flatbuffers,google/flatbuffers,google/flatbuffers,alexames/flatbuffers,alexames/flatbuffers,google/flatbuffers,google/flatbuffers,google/flatbuffers,alexames/flatbuffers,google/flatbuffers,google/flatbuffers,google/flatbuffers,alexames/flatbuffers,alexames/flatbuffers,alexames/flatbuffers,alexames/flatbuffers,alexames/flatbuffers,alexames/flatbuffers,alexames/flatbuffers,alexames/flatbuffers,alexames/flatbuffers,alexames/flatbuffers,google/flatbuffers,alexames/flatbuffers