Using 3rd party system libraries (zlib, harfbuzz, freetype, libpng, libjpeg, openssl) cross-compiled to emscripten cannot be configured as system libraries them even though
there are appropriate Findxzy.cmake files in the provided CMAKE_PREFIX_PATH / CMAKE_MODULE_PATH.
 
Following error result for example with zlib:

ERROR: Feature "system_zlib": Forcing to "ON" breaks its condition:     WrapZLIB_FOUND Condition values dump:     WrapZLIB_FOUND = "FALSE"  

And also these even if configured as system:

   JPEG ................................. yes

     Using system libjpeg ............... no

 FreeType ............................... yes

   Using system FreeType ................ no

 PCRE2 .................................. yes

   Using system PCRE2 ................... no

 WEBP ................................... yes

   Using system libwebp ................. no