cmake: misc improvements, add LIBSSH2_USE_PKGCONFIG option

- show platform flags (via curl).
- add `LIBSSH2_USE_PKGCONFIG` option to control whether to use
  `pkg-config` to find dependencies.
- set `.pc` names withing the Find modules.
- add `mbedcrypto` to `libssh2.pc` only when detected via `pkg-config`.
  Workaround for older mbedtls versions and non-CMake mbedTLS builds
  (as of mbedTLS 3.6.2) that don't emit an `mbedcrypto.pc` file.
- set header paths relative to the project root (tidy-up).
- use `-isystem` for crypto backend and zlib header paths.
  To match autotools.
- sync header path order with autotools.
- rename local variables to underscore-lowercase.
- minor tidy-ups.

Cherry-picked from #1484
Closes #1525

cmake/FindMbedTLS.cmake

@@ -15,20 +15,25 @@
 # - `MBEDTLS_INCLUDE_DIRS`:  The mbedTLS include directories.
 # - `MBEDTLS_LIBRARIES`:     The mbedTLS library names.
 # - `MBEDTLS_LIBRARY_DIRS`:  The mbedTLS library directories.
+# - `MBEDTLS_PC_REQUIRES`:   The mbedTLS pkg-config packages.
 # - `MBEDTLS_CFLAGS`:        Required compiler flags.
 # - `MBEDTLS_VERSION`:       Version of mbedTLS.
 
-if((UNIX OR VCPKG_TOOLCHAIN OR (MINGW AND NOT CMAKE_CROSSCOMPILING)) AND
+set(MBEDTLS_PC_REQUIRES "mbedcrypto")
+
+if(LIBSSH2_USE_PKGCONFIG AND
    NOT DEFINED MBEDTLS_INCLUDE_DIR AND
    NOT DEFINED MBEDCRYPTO_LIBRARY)
   find_package(PkgConfig QUIET)
-  pkg_check_modules(MBEDTLS "mbedcrypto")
+  pkg_check_modules(MBEDTLS ${MBEDTLS_PC_REQUIRES})
 endif()
 
 if(MBEDTLS_FOUND)
   string(REPLACE ";" " " MBEDTLS_CFLAGS "${MBEDTLS_CFLAGS}")
   message(STATUS "Found MbedTLS (via pkg-config): ${MBEDTLS_INCLUDE_DIRS} (found version \"${MBEDTLS_VERSION}\")")
 else()
+  set(MBEDTLS_PC_REQUIRES "")
+
   find_path(MBEDTLS_INCLUDE_DIR NAMES "mbedtls/version.h")
   find_library(MBEDCRYPTO_LIBRARY NAMES "mbedcrypto" "libmbedcrypto")