fix: correctly use CMake's FindX11

[shenlebantongying]

I was thinking about how would this line in #1369 fixes any problem.

The weird thing is that we already linked against X11

goldendict-ng/CMake_Unix.cmake

Lines 14 to 17 in c15cbbf

	find_package(X11 REQUIRED)
	pkg_check_modules(LIBXTST IMPORTED_TARGET xtst)
	target_compile_definitions(${GOLDENDICT} PUBLIC HAVE_X11)
	target_link_libraries(${GOLDENDICT} PRIVATE X11 PkgConfig::LIBXTST)

Combined with this comment #1292 (comment) , I came to realize a stupid mistake I made:

The FindX11.cmake will define X11_LIBRARIES and X11_INCLUDE_DIR but not X11

https://cmake.org/cmake/help/latest/module/FindX11.html

The working of target_link_libraries(${GOLDENDICT} PRIVATE X11 is pure accident because X11 will be translated into -lX11 😅

This PR thanks to @KonstantinDjairo & @Canvis-Me

[sonarcloud]

Quality Gate passed

Kudos, no new issues were introduced!

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See analysis details on SonarCloud

[KonstantinDjairo]

tested in my freebsd machine, works perfectly
thank you for refactoring it