CMake: Remove need to fiddle with CMAKE_C_FLAGS / CMAKE_CXX_FLAGS

[mwtoews]

This PR is to help modernize the CMake setup (#1263)

There a are likely several ways to accomplish this, but this is the cleanest way that I can see. Most of the custom flags added to date are warning flags. There are now cached into PROJ_C_WARN_FLAGS/PROJ_CXX_WARN_FLAGS, then applied to the relevant targets.

Currently, these target compile options PRIVATE, since warning flags should not change the behavior of the library/utilities. But perhaps these should be INTERFACE?

Also, /D_CRT_SECURE_NO_WARNINGS is not exactly a warning flag, so it's been bumped into a global add_definitions.

The only remaining CMAKE_C_FLAGS modification is related to an Intel flag, which I may address another day.

[mwtoews]

Oh, nearly forgot to mention moving the interprocedural optimization flag -flto as a target compile option. It is currently set as PRIVATE, but should it be PUBLIC or INTERFACE instead? From what I understand, this option is both a compile and linker flag.

The two failures are related to implicit function declaration, which changed between C89 and C99 Standards. PR #1410 makes it bit more clear to see the compile command to help sort out this type of situation.

[mwtoews]

The two C89 failures are now sorted, which needed "all warnings" added to CMAKE_REQUIRED_FLAGS (I'm nearly certain there is a CMake bug with check_c_source_compiles, as the C99 example clearly fails with -std=c89; also, CMake has >2k open issues, so I'd rather not dig any deeper).

This PR is ready for review. I.e., are there any opinions on the use of PRIVATE vs. PUBLIC or INTERFACE as mentioned above for the scope of warning flags or IPO?

[kbevers]

This PR is ready for review. I.e., are there any opinions on the use of PRIVATE vs. PUBLIC or INTERFACE as mentioned above for the scope of warning flags or IPO?

Overall I think this is great. I have no idea what is the right thing to do regarding the PRIVATE/PUBLIC thing. What will be different if PUBLIC is chosen instead of PRIVATE?

[mwtoews]

PRIVATE is the easiest option to go with, as these flags only go to those targets, and don't get imported or passed around to others.

Warning flags are generally added to targets with PRIVATE (e.g. here, here). However, I'm aware the folks in the libgeos camp (e.g. dbaston's modern-cmake) have chosen to go with INTERFACE for warnings, and generally have a different approach to applying the flags to targets (for instance, I generally find if-conditions more readable than generator expressions).

As for IPO/LTO, PRIVATE produces the same compile commands as existing, so this selection is probably fine. But in the CMake realm, I'm not sure if those properties need to be retained if imported/used by other targets, since it's a compiler and linker flag.

[kbevers]

After reading this I think that PRIVATE is the right option here. It would be interesting to know why GEOS chose to use INTERFACE over PRIVATE though.

[mwtoews]

Yup, that makes sense. Nevertheless, the scope keyword is simple to toggle, if there is a good argument to be made.

[kbevers]

Right, let's just go with what you've made so far. We'll change it if is needed at some point in the future.