Reflect directory structure of includes in ${PROJECT_BINARY_DIR}/out/inc

[thedmd]

Configured headers were flattened during configuration. This patch
keep original directory structure.

For example without this for 'dequeis replaced byexperimental/deque`.

Please note that acceptance of community PRs will be delayed while we are
bringing our test and CI systems online. For more information, see the
README.md.

Description

Configured headers were flattened during configuration rendering build broken
as some headers were replaced by counterparts in subdirectories.
For example without this for 'dequeis replaced byexperimental/deque`.

Checklist:

• I understand README.md.
• If this is a feature addition, that feature has been voted into the C++
  Working Draft.
• Any code files edited have been processed by clang-format.
• Identifiers in any product code changes are properly _Ugly as per
  https://eel.is/c++draft/lex.name#3.1 .
• Identifiers in test code changes are not _Ugly.
• Test code includes the correct headers as per the Standard, not just
  what happens to compile.
• The STL builds and test harnesses have passed (must be manually verified
  by an STL maintainer before CI is online, leave this unchecked for initial
  submission).
• This change introduces no known ABI breaks (adding members, renaming
  members, adding virtual functions, changing whether a type is an aggregate or
  trivially copyable, etc.). If unsure, leave this box unchecked and ask a
  maintainer for help.

Note: This is change in CMakeLists.txt only, C++ code was not touched

[CaseyCarter]

Well, this is embarrassing. I can confirm that the bug exists, and that this change fixes it, but I'd like one of our cmake experts to sign off on the exact form of the fix before merging. (Paging @BillyONeal, @barcharcraz.)

[BillyONeal]

"expert" is a strong word in my case! :)

@thedmd Thanks for your contribution!