graphics/gdal: Document CMAKE_CXX_SCAN_FOR_MODULES in d8feecdf7fdb6514ad4b714e69b2045d8c297d52

C++ 20 introduced the concept of "modules" to the language. The design requires
build systems to order compilations among each other to satisfy import
statements reliably. CMake's implementation asks the compiler to scan source
files for module dependencies during the build, collates scanning results to
infer ordering constraints, and tells the build tool how to dynamically update
the build graph.

cmake-cxxmodules(7) was added in cmake 3.28. It will scan for c++ modules for
c++20 by default.

Since poppler uses c++20, it triggers cmake to scan modules by default and it
requires clang-scan-deps from llvm.

Therefore, we need to turn off CMAKE_CXX_SCAN_FOR_MODULES to fix the build and
avoid adding extra dependency on llvm.

Reference: https://cmake.org/cmake/help/latest/manual/cmake-cxxmodules.7.html