cmake: cleanup header paths

- sync build-dir/source-dir header path order with autotools, by
  including build-dir first, then source-dir.
  This prevents out-of-tree builds breaking due to leftover generated
  headers in the source tree.

- tests/unit: move `src` ahead of `libtest` in header path, syncing with
  autotools.

- stop adding non-existing generated `include` dir to header path.
  There are no generated `include` headers and this directory is either
  missing in out-of-tree builds or the same as the one already added
  globally via the root `CMakeLists.txt`.

- lib: stop adding a duplicate source include directory to the header
  path.
  It's already added globally via the root `CMakeLists.txt`.

- lib: stop adding the project root to the header path.

- docs/examples: drop internal header paths.
  Examples do not and should not use internal headers.

- replace `curl_setup_once.h` in comments with `curl_setup.h`,
  the header actually used, and also referred to in autotools comments.

- add comment why we need `src` in include path for `tests/server`.

- add quotes around header directories.

Closes #14416

docs/examples/CMakeLists.txt

@@ -30,11 +30,6 @@ include(${CMAKE_CURRENT_BINARY_DIR}/Makefile.inc.cmake)
 foreach(_example IN LISTS check_PROGRAMS)
   set(_example_target "curl-example-${_example}")
   add_executable(${_example_target} "${_example}.c")
-  target_include_directories(${_example_target} PRIVATE
-    "${CURL_SOURCE_DIR}/lib"      # for "curl_setup_once.h"
-    "${CURL_BINARY_DIR}/lib"      # for "curl_config.h"
-    "${CURL_BINARY_DIR}/include"  # for "curl/curl.h"
-  )
   target_link_libraries(${_example_target} ${LIB_SELECTED} ${CURL_LIBS})
   target_compile_definitions(${_example_target} PRIVATE "CURL_NO_OLDIES")
   if(LIB_SELECTED STREQUAL LIB_STATIC AND WIN32)