include FindGDAL.cmake in OTB/CMake
"find_package(GDAL REQUIRED)" will fail on linux distributions (not all) if configuring projects using OTB with binary packages. This is due to fact that gdal headers are installed in some distro such as ubuntu in "/include/gdal" while package is installing it in "/include".
Either we have to patch superbuild to install gdal into "include/gdal" (not recommended) or keep a copy of "FindGDAL.cmake" in src tree.
Patching superbuild to change include directory can work for ubuntu in this case but may break on other systems. Adding FindGDAL.cmake into src tree we can instruct cmake to look into "include" and also look into "include/gdal". So including this file seems practical solution for now. Even if we try to patch cmake findgdal.cmake we still have "N" number of cmake versions (OTB supports cmake version since 3.0) out there that's gonna mess up a user installation. Also GDAL is important dependency in projects using OTB. Many of them use gdal apps and api directly without going through GDALImageIO
upstream version of cmake: https://github.com/Kitware/CMake/blob/master/Modules/FindGDAL.cmake