make FastPFor easier to use as submodule

[deafwolf]

fixes some cmake error and compile warning

• ${CMAKE_MODULE_PATH}/simde.cmake may cause simde not found if use it as module
• the size_t i shadow the variable i out of the loop
• set CXX_FLAG and C_FLAG to avoid error when cmake -DCMAKE_BUILD_TYPE=RelWithDebInfo

[lemire]

set CXX_FLAG and C_FLAG to avoid error when cmake -DCMAKE_BUILD_TYPE=RelWithDebInfo

Everything looks fine, but this part I do not understand.

If you want a Release build, why don't you type cmake -DCMAKE_BUILD_TYPE=Release instead?

[lemire]

I am going to merge, but delete the set RelWithDebInfo flags as release part. If you need it, please issue a PR with an explanation.