if you configure and build with zstd installed and on path, it automatically detects and uses. FINE/GREAT.

But the .pc file generated on windows (
mongoc2-static.pc) is bad.
I fixed/workaround with:
 
    @# FIX:
    @#      Requires: bson2-static
    @#      Libs: ${libdir}/mongoc2.lib -lDnsapi secur32.lib crypt32.lib Bcrypt.lib ncrypt.lib -lC:/Sandbox/Stroika/DevRoot/Builds/Debug/lib/zstd.lib
    @#  to
    @#      Requires: bson2-static, libzstd
    @#      Libs: ${libdir}/mongoc2.lib -lDnsapi secur32.lib crypt32.lib Bcrypt.lib ncrypt.lib
    @#
    @${StroikaRoot}ScriptsLib/PrintProgressLine $$((${MAKE_INDENT_LEVEL}+1)) "mongo-cxx-driver ${MONGOCXXDRIRVER_VERSION_} - Patching: ${PER_CONFIGURATION_BUILDS_DIR_}/lib/pkgconfig/mongoc2-static.pc"
    @if grep -q "-l[^ ]*zstd\.lib" "${PER_CONFIGURATION_BUILDS_DIR_}/lib/pkgconfig/mongoc2-static.pc"; then\
        ${StroikaRoot}ScriptsLib/PrintProgressLine $$((${MAKE_INDENT_LEVEL}1)) '';\
        ${SED} -i "s/-l[^ ]*\/zstd\.lib//" "${PER_CONFIGURATION_BUILDS_DIR_}/lib/pkgconfig/mongoc2-static.pc";\
        ${StroikaRoot}ScriptsLib/PrintProgressLine $$((${MAKE_INDENT_LEVEL}1)) '';\
        ${SED} -i "s/Requires: .*/&, libzstd/" "${PER_CONFIGURATION_BUILDS_DIR_}/lib/pkgconfig/mongoc2-static.pc";\
    fi