CMake: Fix a misplaced > in pkg-config files

The Qt CMake routines for generating pkg-config files don't
handle all sorts of generator expressions, see
qt_internal_set_pkg_config_cpp_flags in QtPkgConfigHelpers.cmake
which hardcodes handling of some specific expressions. In this
case, they don't handle the semicolon within the generator
expression expansion.

For the UNICODE and _UNICODE defines, this means that the pkg-config
file ends up containing "-D_UNICODE>" with the trailing ">". (The
pkg-config generator tries to parse out the generator expressions,
but the semicolon gets handled as a higher level separator, leaving
the closing bracket ">" behind.)

This issue only shows up for mingw targets, because pkg-config files
aren't generated in MSVC style builds.

Escape the semicolon as $<SEMICOLON> to make it not break the
surrounding generator expression, as parsed by the pkg-config file
generator.

The generator expressions aren't fully correctly evaluated for the
pkg-config files though; the UNICODE and _UNICODE defines don't
end up in the resulting pkg-config file even though they're used
during compilation (both before and after this change).

Fixes: QTBUG-103019
Co-authored-by: Martin Reboredo <yakoyoku@gmail.com>
Pick-to: 6.5 6.2
Change-Id: Icdb380e3b42be2a47372a8ee2b61378a33c685f7
Reviewed-by:  Alexey Edelev <alexey.edelev@qt.io>
Reviewed-by: Li Xinwei <1326710505@qq.com>
(cherry picked from commit f35b9530b9acf954f8741d0ee2b4b68fc90a4afc)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

cmake/QtFlagHandlingHelpers.cmake

@@ -404,7 +404,7 @@ function(qt_internal_enable_unicode_defines)
         set(no_unicode_condition
             "$<NOT:$<BOOL:$<TARGET_PROPERTY:QT_NO_UNICODE_DEFINES>>>")
         target_compile_definitions(Platform
-            INTERFACE "$<${no_unicode_condition}:UNICODE;_UNICODE>")
+            INTERFACE "$<${no_unicode_condition}:UNICODE$<SEMICOLON>_UNICODE>")
     endif()
 endfunction()