cmake: untangle feature detection interdependencies

[vszakats]

• reduce check_include_file_concat() use to those headers that either
  depend on a previously detected header, or another header or symbol
  detection depend on it.

• replace check_symbol_exists() with check_function_exists() for
  functions that are detected with AC_CHECK_FUNCS() in ./configure.
  This makes setmode() no longer be detected with MSYS, syncing
  this with ./configure. Instead _setmode() is used now also in
  CMake MSYS builds. This is consistent with Cygwin builds also.

• add comment about which header/symbol detection depends on what
  header. Based on ./configure mainly.

• form CURL_TEST_DEFINES manually, and include only those macros which
  are actually used in CMake/CurlTests.c.

• change curl_internal_test() to use CMAKE_REQUIRED_DEFINITIONS,
  instead of CMAKE_REQUIRED_FLAGS to simplify the logic, and to allow
  dropping the latter macro completely.

• drop windows.h from header and symbol checks.

• ./configure: add comment about whether netinet/in6.h, sys/un.h
  are indeed meant to be included for all detections. There is a chance
  they were added there by accident.

Detection resuls were cross-checked between
436bbbe (master) and
48ff4694e608ccfdedf7ce5bab2b96d6b2c23cda (this PR), for CI GHA Linux,
Linux HTTP/3, non-native, macOS and Windows jobs.

---

• rebase on cmake: replace check_include_file_concat() for LDAP and GSS detection #15157
• revert setmode detection to be function-based, add _setmode detection for Cygwin/MSYS.
• split symbol/function checks into two lists and sort them.
• do the same for pre-cached feature flags in WindowsCache.cmake.

[vszakats]

Script used to extract detection results from GHA logs:

#!/bin/sh
grep -R -E '(\#define HAVE_| checking |-- Checking |-- Check |-- Performing |-- Looking )' \
| grep -v -F ' | ' | grep -v -F 'configure:' \
| sed -E \
  -e 's/:[0-9]{4}-[0-9]{2}-[0-9]{2}T[0-9]{2}:[0-9]{2}:[0-9]{2}.[0-9]{7}Z /: /g' \
  -e 's|./logs_[0-9]{11}/([0-9]{1,2}_)?||g' \
  -e 's/ [0-9]+ include files [a-zA-Z0-9._/-]+, \.\.\., / /g' \
  -e 's/ for include files [a-zA-Z0-9._/-]+, / for /g' \
  -e 's/ for include files? / for /g' \
  | sort

This will become simpler after #15266.