Fix CMake checks for Linux

The LINUX variable is not a thing (at least not by default).
Use an explicit CMAKE_SYSTEM_NAME check.

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index d171a67..6b4cf08 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -222,7 +222,7 @@
 endif()
 
 # The version of clang currently on our Mac bots doesn't seem to support this flag.
-if (CMAKE_CXX_COMPILER_ID MATCHES "Clang" AND (LINUX OR WINDOWS))
+if (CMAKE_CXX_COMPILER_ID MATCHES "Clang" AND NOT APPLE)
   add_link_flag("-fuse-ld=lld")
 endif()
 
@@ -473,7 +473,7 @@
 else()
   message(STATUS "Building libbinaryen as shared library.")
   add_library(binaryen SHARED)
-  if(LINUX)
+  if(CMAKE_SYSTEM_NAME STREQUAL "Linux")
     # Disable interposition and resolve Binaryen symbols locally.
     add_link_flag("-Bsymbolic")
   endif()