GHA/configure-vs-cmake: check libcurl.pc/curl-config, fix issues

Add CI checker to compare `libcurl.pc` and `curl-config` files
generated by autotools and cmake builds.

Fix differences and apply tiny cleanups:
- curl-config: use single-quotes for literals.
- curl-config: quote all variables.
- curl-config: replace double with single quotes in a substituted value
  that's always literal (`@prefix@`).
- libcurl.pc: spelling in `Description:`.
- libcurl.pc: avoid substitution in a comment.
- cmake: fill `libdir` with `${exec_prefix}` instead of a literal.
  To sync with './configure'.
- configure: fix `CURL_CA_BUNDLE` value to not generate nested quotes
  in `curl-config`.
- configure: add missing `LDFLAGS` to `Libs.private` in `libcurl.pc`.
  To sync with CMake.
- cmake: skip adding `CMAKE_C_IMPLICIT_LINK_LIBRARIES` for MINGW and
  UNIX. They added these values as seen in CI:
  MINGW: `-lmingw32 -lgcc -lmoldname -lmingwex -ladvapi32 -lshell32 -luser32 -lkernel32 -lmingw32 -lgcc -lmoldname -lmingwex`
  Linux: `-lgcc -lgcc_s -lc -lgcc -lgcc_s`
- cmake: delete FIXME about enabling libssh2 by default.
  `./configure` has special defaults for these packages (called: "off"):
  brotli, zstd, libpsl, libssh2, libssh, wolfssl, librtmp
  It looks for them, but only at system locations, which makes them
  never detected e.g. on macOS. CMake doesn't offer such default mode
  for now.
- GHA/macos: drop now redundant `-DCURL_DISABLE_LDAPS=ON`.
- cmake: use `CMAKE_INSTALL_INCLUDEDIR` and `CMAKE_INSTALL_LIBDIR`
  instead of hardcoded `include`/`lib` when generating `libcurl.pc`.

Updates to the GHA workflow:
- move autotools out-of-tree and rename cmake out-of-tree directory
  to `bld_cm` to tell it's cmake.
- disable static libcurl for `./configure` to match cmake.
- enable `pkg-config` debug output with `./configure`.
- dump list of Homebrew packages on macOS.
- dump `./configure` detailed logs.
- disable zstd and brotli for Linux, to match cmake.

There remain differences, mostly due to detection order and method. Also
some values are inherently different when using CMake and autotools,
such as `--cc`, `--configure`. autotools also generates duplicates for
`-lssl` and `-lcrypto`. macOS LDAP wants to link `-lber` while autotools
doesn't. Some build defaults are also different in autotools and cmake.
These differences are smoothened out for now by the checker script, or
via build options. Notice that lib order (a dupes) _can_ be significant
in some cases. E.g. the binutils linker is infamous for that on Windows.

Closes #14681

libcurl.pc.in

@@ -31,11 +31,11 @@ supported_features="@SUPPORT_FEATURES@"
 
 Name: libcurl
 URL: https://curl.se/
-Description: Library to transfer files with ftp, http, etc.
+Description: Library to transfer files with HTTP, FTP, etc.
 Version: @CURLVERSION@
 Requires: @LIBCURL_PC_REQUIRES@
 Requires.private: @LIBCURL_PC_REQUIRES_PRIVATE@
 Libs: -L${libdir} -lcurl @LIBCURL_PC_LIBS@
-Libs.private: @LIBCURL_PC_LIBS_PRIVATE@
+Libs.private: @LDFLAGS@ @LIBCURL_PC_LIBS_PRIVATE@
 Cflags: -I${includedir} @LIBCURL_PC_CFLAGS@
 Cflags.private: @LIBCURL_PC_CFLAGS_PRIVATE@