Revert "SCons: Add CPPEXTPATH for external includes"

[Repiteo]

• Reverts SCons: Add CPPEXTPATH for external includes (reverted) #104893

Despite looking promising on-paper, this change proved to be much more trouble than it's worth. The reason for that was revealed when working on #108611, as it came down to SCons not tracking external includes properly. While that might've sufficed in isolation, this fix required a somewhat invasive and largely undocumented solution. Combined with it being in a stagnant/unreviewed state, the much safer option is to simply revert to the original implementation

This isn't a full revert, as there've been hotfixes surrounding this initial change since merging, so those are already applied. Beyond that, the sections that necessitated warning suppression pragmas now use the warning wrapper macros instead, as those would've been applied if they were still around

Note that this reimplements hardcoded -isystem flags to the repo, as those were around prior to this original PR. Meaning: the same issues that CPPEXTPATH revealed had already existed in the repo for many years, but either never came up or were addressed in an isolated context. As such, a followup PR will remove these as well, and apply the proper warning wrappers to the thirdparty includes as necessary

Bugsquad edit:

• Different fix for Can no longer build godot and godot-nir-static from source together #106376
• Fixes this issue: Jolt: Update to 5.4.0 #110965 (comment)
• Fixes SCons: Add CPPEXTPATH for external includes (reverted) #104893 (comment)
• Supersedes Vulkan: Add macros to avoid compilation issues with missing symbols in Vulkan 1.0 #111094

[akien-mga]

Looks good to me.

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 4.5.1.