Update snappy to 1.1.8 and disable building tests for it

Disabling the building of tests is a workaround for issue #1791 and is
probably not an issue since snappy tests are not executed as part of the
build process of orc c++ libs.

Note that snappy 1.1.9 and 1.1.10 cannot be used as is because source
assets on snappy's gh page are not buildable out of the box, so stick to
1.1.8 for now.

cmake_modules/ThirdpartyToolchain.cmake

@@ -17,7 +17,7 @@
 
 set(ORC_FORMAT_VERSION "1.0.0")
 set(LZ4_VERSION "1.9.3")
-set(SNAPPY_VERSION "1.1.7")
+set(SNAPPY_VERSION "1.1.8")
 set(ZLIB_VERSION "1.2.11")
 set(GTEST_VERSION "1.12.1")
 set(PROTOBUF_VERSION "3.5.1")
@@ -99,7 +99,7 @@ else ()
 
   ExternalProject_Add (snappy_ep
     URL "https://github.com/google/snappy/archive/${SNAPPY_VERSION}.tar.gz"
-    CMAKE_ARGS ${SNAPPY_CMAKE_ARGS}
+    CMAKE_ARGS ${SNAPPY_CMAKE_ARGS} -DSNAPPY_BUILD_TESTS=OFF
     ${THIRDPARTY_LOG_OPTIONS}
     BUILD_BYPRODUCTS "${SNAPPY_STATIC_LIB}")