compat/nedmalloc: Fix compiler warnings on linux

On linux, when the build variable USE_NED_ALLOCATOR is set, gcc issues the following warnings: In file included from compat/nedmalloc/nedmalloc.c:63: .../malloc.c.h: In function 'mmap_resize': .../malloc.c.h:3762: warning: implicit declaration of function 'mremap' .../malloc.c.h: In function 'sys_trim': .../malloc.c.h:4195: warning: comparison between pointer and integer The warnings are caused by the <sys/mman.h> header not enabling the (conditional) declaration of the mremap() function. The declaration can be enabled by defining the _GNU_SOURCE symbol prior to including certain system header files. In particular, it may not be sufficient to simply define _GNU_SOURCE just prior to including the <sys/mman.h> header. (e.g. defining the symbol after including <sys/types.h> will be completely ineffective.) In order to suppress the warnings, we define the _GNU_SOURCE symbol at the start of the malloc.c.h header file. Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk> Signed-off-by: Junio C Hamano <gitster@pobox.com>