build: replace `string` with JOIN option to `string_join` helper in CMake

`string` with JOIN option is only available since CMake 3.12, but we have
developers using CMake 3.1: implement a utility `string_join` function to
remove this dependency.

Closes #5881

NO_CHANGELOG=<build fix>
NO_DOC=<build fix>
NO_TEST=<build fix>

(cherry picked from commit 55298308)

cmake/compiler.cmake

+include(cmake/utils.cmake)
+
 #
 # Check if the same compile family is used for both C and CXX
 #
@@ -240,7 +242,7 @@ macro(enable_tnt_compile_flags)
         if (NOT CMAKE_COMPILER_IS_CLANG)
             message(FATAL_ERROR "Undefined behaviour sanitizer only available for clang")
         else()
-            string(JOIN "," SANITIZE_FLAGS
+            string_join("," SANITIZE_FLAGS
                 alignment bool bounds builtin enum float-cast-overflow
                 float-divide-by-zero function integer-divide-by-zero return
                 shift unreachable vla-bound

cmake/utils.cmake

@@ -114,3 +114,21 @@ function(file_is_in_directory varname file dir)
         set(${varname} TRUE PARENT_SCOPE)
     endif()
 endfunction()
+
+# string() has JOIN option but only since 3.12.
+function(string_join
+    glue
+    string_out
+)
+    set(result "")
+    set(is_first_item TRUE)
+    foreach(item IN LISTS ARGN)
+        if(is_first_item)
+            set(result "${item}")
+            set(is_first_item FALSE)
+        else()
+            set(result "${result}${glue}${item}")
+        endif()
+    endforeach()
+    set(${string_out} ${result} PARENT_SCOPE)
+endfunction()