EigenTesting.cmake: Quote argument to separate_arguments.

libeigen/eigen!2077

Closes #3005 and #2866

cmake/EigenTesting.cmake

diff --git a/cmake/EigenTesting.cmake b/cmake/EigenTesting.cmake
index d140fe6..2a44f05 100644
--- a/cmake/EigenTesting.cmake
+++ b/cmake/EigenTesting.cmake
@@ -75,7 +75,7 @@
 
   # let the user pass flags.
   if(${ARGC} GREATER 2)
-    separate_arguments(compile_options NATIVE_COMMAND ${ARGV2})
+    separate_arguments(compile_options NATIVE_COMMAND "${ARGV2}")
     target_compile_options(${targetname} PRIVATE ${compile_options})
   endif()