ClamAV: only require libintl if building for Linux.

This fixes build failures on Darwin caused by 274b6e84d9b3a77e76cc2a49053eba985ef62045
since we don't have that GNU library available. The benchmark seems to guard
requiring the header under Linux, so I've done the same in the cmake file.
diff --git a/MultiSource/Applications/ClamAV/CMakeLists.txt b/MultiSource/Applications/ClamAV/CMakeLists.txt
index 849b773..217c725 100644
--- a/MultiSource/Applications/ClamAV/CMakeLists.txt
+++ b/MultiSource/Applications/ClamAV/CMakeLists.txt
@@ -170,6 +170,8 @@
   zlib_uncompr.c
   zlib_zutil.c
 )
-find_package(Intl REQUIRED)
-target_link_libraries(clamscan ${Intl_LIBRARIES})
+if(TARGET_OS STREQUAL "Linux")
+  find_package(Intl REQUIRED)
+  target_link_libraries(clamscan ${Intl_LIBRARIES})
+endif()
 llvm_test_data(clamscan ${INPUT} dbdir)