Add find module for Snappy

Summary:
Add a find_package module for Snappy
Closes https://github.com/facebook/folly/pull/783

Reviewed By: simpkins

Differential Revision: D7079554

Pulled By: yfeldblum

fbshipit-source-id: bf5dc3f3c37367f43fcbf8b40aaa3bb0a030bee5

CMake/FindSnappy.cmake

+# Find the Snappy libraries
+#
+# This module defines:
+# SNAPPY_FOUND
+# SNAPPY_INCLUDE_DIR
+# SNAPPY_LIBRARY
+
+find_path(SNAPPY_INCLUDE_DIR NAMES snappy.h)
+find_library(SNAPPY_LIBRARY NAMES snappy)
+
+include(FindPackageHandleStandardArgs)
+FIND_PACKAGE_HANDLE_STANDARD_ARGS(
+    SNAPPY DEFAULT_MSG
+    SNAPPY_LIBRARY SNAPPY_INCLUDE_DIR
+)
+
+mark_as_advanced(SNAPPY_INCLUDE_DIR SNAPPY_LIBRARY)

CMake/folly-deps.cmake

@@ -112,9 +112,11 @@ if(ZSTD_FOUND)
   list(APPEND FOLLY_LINK_LIBRARIES ${ZSTD_LIBRARY})
 endif()
 
-CHECK_INCLUDE_FILE_CXX(snappy.h FOLLY_HAVE_LIBSNAPPY)
-if (FOLLY_HAVE_LIBSNAPPY)
-  list(APPEND FOLLY_LINK_LIBRARIES snappy)
+find_package(Snappy MODULE)
+set(FOLLY_HAVE_LIBSNAPPY ${SNAPPY_FOUND})
+if (SNAPPY_FOUND)
+  list(APPEND FOLLY_INCLUDE_DIRECTORIES ${SNAPPY_INCLUDE_DIR})
+  list(APPEND FOLLY_LINK_LIBRARIES ${SNAPPY_LIBRARY})
 endif()
 
 find_package(LibDwarf)