configure: use .altmacro for gnu as check on arm

Clang's integrated assembler (after 3.4) does not yet support the
'.altmacro' directive which is only used in arm asm. Support is planned:
http://llvm.org/bugs/show_bug.cgi?id=18918
av_dict_set: fix potential memory leak with AV_DICT_DONT_OVERWRITE

av_dict_set leaks it key/value arguments if AV_DICT_DONT_OVERWRITE is
combined with AV_DICT_DONT_STRDUP_{KEY,VAL} and the key exists.