fix compilation in debug mode with Clang

On my machine, Clang cannot compile the code in debug mode.

    quadiron/src/arith.h:299: undefined reference to `__muloti4'
    quadiron/src/arith.h:303: undefined reference to `__muloti4'
    quadiron/src/arith.h:307: undefined reference to `__muloti4'

This seems related to this bug: https://bugs.llvm.org/show_bug.cgi?id=16404

Removing `-ftrapv` for Clang solves the issue.

CMakeLists.txt

@@ -153,7 +153,6 @@ endforeach(flag)
 # Debug flags
 set(DEBUG_CXX_FLAGS
   -O0 -g3 -ggdb3 -fno-limit-debug-info
-  -ftrapv
 )
 foreach(flag ${DEBUG_CXX_FLAGS})
   check_cxx_compiler_flag(${flag} has_flag_${flag})
@@ -162,6 +161,11 @@ foreach(flag ${DEBUG_CXX_FLAGS})
   endif(has_flag_${flag})
 endforeach(flag)
 
+# Seems buggy on Clang: see https://bugs.llvm.org/show_bug.cgi?id=16404
+if (CMAKE_CXX_COMPILER_ID MATCHES "GNU")
+    set(CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} -ftrapv")
+endif()
+
 # Release flags
 set(RELEASE_CXX_FLAGS
   -O3 -DNDEBUG