Comment # 12 on bug 1000513 from Jiri Slaby

And to fill the gap, 4.9 is still OK:
$ gcc-4.9 dw.c -o dw.o -g -c -O2 && readelf -wi dw.o|grep -C 5 getenv
    <2fb>   DW_AT_type        : <0x264>
    <2ff>   DW_AT_external    : 1
    <2ff>   DW_AT_declaration : 1
 <1><2ff>: Abbrev Number: 19 (DW_TAG_subprogram)
    <300>   DW_AT_external    : 1
    <300>   DW_AT_name        : (indirect string, offset: 0x268): getenv
    <304>   DW_AT_decl_file   : 6
    <305>   DW_AT_decl_line   : 564
    <307>   DW_AT_prototyped  : 1
    <307>   DW_AT_type        : <0x8b>
    <30b>   DW_AT_declaration : 1

So it has to be something new in gcc-5.