linking fails (missing `-lm`)

[claudeha]

As a CMake neophyte I'm not sure this is the best solution but it works for me:

diff --git a/ray4/CMakeLists.txt b/ray4/CMakeLists.txt
index 79cf8c9..a05d896 100644
--- a/ray4/CMakeLists.txt
+++ b/ray4/CMakeLists.txt
@@ -21,3 +21,4 @@ set ( sources_ray4
 )
 
 add_executable (ray4 ${sources_ray4})
+target_link_libraries(ray4 -lm)