[cmake] Pass only one sanitizer to TFEL, prefer ASAN.

Multiple flags required to be passed whitespace-separated and this
messes up CMake's external project builds. Could not get it to work.

scripts/cmake/DependenciesExternalProject.cmake

@@ -81,17 +81,19 @@ if(OGS_USE_MFRONT)
             )
         endif()
 
+        # Only one flag supported, prefer ASAN
         if(ENABLE_ASAN)
-            list(APPEND _sanitize_flags -fsanitize=address)
+            set(_sanitize_flag -fsanitize=address)
         endif()
-        if(ENABLE_UBSAN)
-            list(APPEND _sanitize_flags -fsanitize=undefined)
+        if(ENABLE_UBSAN AND NOT DEFINED _sanitize_flag)
+            set(_sanitize_flag -fsanitize=undefined)
+        elseif(ENABLE_UBSAN AND DEFINED _sanitize_flag)
+            message(STATUS "MFront: ASAN enabled only! UBSAN is off.")
         endif()
-        if(DEFINED _sanitize_flags)
-            list(JOIN _sanitize_flags " " _sanitize_flags_string)
+        if(DEFINED _sanitize_flag)
             foreach(var CXX EXE_LINKER SHARED_LINKER MODULE_LINKER)
                 list(APPEND _tfel_cmake_args
-                     "-DCMAKE_${var}_FLAGS_INIT=${_sanitize_flags_string}"
+                     "-DCMAKE_${var}_FLAGS_INIT=${_sanitize_flag}"
                 )
             endforeach()
         endif()
@@ -103,7 +105,7 @@ if(OGS_USE_MFRONT)
                        "-DCMAKE_POSITION_INDEPENDENT_CODE=ON"
                        "-Denable-testing=OFF"
                        ${_defaultCMakeArgs}
-                       ${_tfel_cmake_args}
+                       "${_tfel_cmake_args}"
         )
         message(
             STATUS