cmake: check for empty libsndfile version

for some reason the variable `PC_LIBSNDFILE_VERSION` in our AppVeyor build pipeline is defined but empty. On the local mac I used for testing `libsndfile` was not found using `pkg-config` causing the variable to be not defined at all. In the remote version, the log suggests it is not found by `pkg-config` either. But the `cmake` version used in there falls back to a different behavior causing the variable to be defined but empty.

Checking for an empty string will hopefully work for all platforms

CMakeLists.txt

@@ -358,7 +358,7 @@ set(LIBSNDFILE_VERSION_FLAC_OGG "1.0.17")
 set(LIBSNDFILE_VERSION_OPUS "1.0.28")
 set(LIBSNDFILE_VERSION_MP3 "1.0.31")
 
-if (DEFINED PC_LIBSNDFILE_VERSION)
+if (DEFINED PC_LIBSNDFILE_VERSION AND PC_LIBSNDFILE_VERSION STRGREATER "")
     string(COMPARE GREATER "${PC_LIBSNDFILE_VERSION}" "${LIBSNDFILE_VERSION_FLAC_OGG}" H2CORE_HAVE_FLAC_SUPPORT)
     string(COMPARE GREATER "${PC_LIBSNDFILE_VERSION}" "${LIBSNDFILE_VERSION_OPUS}" H2CORE_HAVE_OPUS_SUPPORT)
     string(COMPARE GREATER "${PC_LIBSNDFILE_VERSION}" "${LIBSNDFILE_VERSION_MP3}" H2CORE_HAVE_MP3_SUPPORT)