configure,cmake: test for -Wstring-conversion

BUG=aomedia:621

Change-Id: I60b4fa9e5ebbef37bf1859a98d82c34821d2d02b

build/cmake/aom_configure.cmake

@@ -177,6 +177,7 @@ else ()
   add_compiler_flag_if_supported("-Wimplicit-function-declaration")
   add_compiler_flag_if_supported("-Wpointer-arith")
   add_compiler_flag_if_supported("-Wsign-compare")
+  add_compiler_flag_if_supported("-Wstring-conversion")
   add_compiler_flag_if_supported("-Wtype-limits")
   add_compiler_flag_if_supported("-Wuninitialized")
   add_compiler_flag_if_supported("-Wunused")

configure

@@ -737,6 +737,7 @@ process_toolchain() {
         check_add_cflags -Wuninitialized
         check_add_cflags -Wunused
         check_add_cflags -Wsign-compare
+        check_add_cflags -Wstring-conversion
         check_add_cflags -Wlogical-op
         check_add_cflags -Wstack-usage=320000
         # Enabling the following warning (in combination with -Wunused above)