Use pkg-config file for cmark-gfm

The upstream-provided cmake installation isn't set up to allow the use of
find_package(), and while vcpkg has a patch for this[1], none of the linux
distributions I've checked carry this. Thankfully, cmark-gfm carries a
pkg-config file, which should be perfect for this situation.

[1]: https://github.com/microsoft/vcpkg/blob/9dd12f8d791f6fa4e396adbebc714d51cbee2143/ports/cmark-gfm/add-feature-tools.patch

Submitted: https://github.com/desktop-app/cmake_helpers/pull/440

--- tdesktop-6.9.3-full.orig/cmake/external/cmark_gfm/CMakeLists.txt
+++ tdesktop-6.9.3-full/cmake/external/cmark_gfm/CMakeLists.txt
@@ -18,6 +18,16 @@
         )
         return()
     endif()
+
+    find_package(PkgConfig)
+    if (PkgConfig_FOUND)
+        pkg_check_modules(DESKTOP_APP_CMARK_GFM IMPORTED_TARGET libcmark-gfm)
+    endif()
+
+    if (DESKTOP_APP_CMARK_GFM_FOUND)
+        target_link_libraries(external_cmark_gfm INTERFACE PkgConfig::DESKTOP_APP_CMARK_GFM)
+        return()
+    endif()
 endif()
 
 # Everything that follows is scoped to this block(). All variables, the