MINGW: config.mak.uname: reorganize MINGW settings

HAVE_LIBCHARSET_H and NO_R_TO_GCC_LINKER are not specific to
msysGit, they're general MinGW settings.

Logic behind HAVE_LIBCHARSET_H: if user is on MinGW and has iconv,
we expect him to have libcharset.h. If user doesn't have iconv,
he has to explicitly say so via NO_ICONV=1 regardless of this
commit.

Signed-off-by: Marat Radchenko <marat@slonopotamus.org>
MINGW: fix main() signature in http-fetch.c and remote-curl.c

On MinGW, compat/mingw.h defines a 'mingw_main' wrapper function.
Fix `warning: passing argument 2 of 'mingw_main' from incompatible
pointer type` in http-fetch.c and remote-curl.c by dropping 'const'.

Signed-off-by: Marat Radchenko <marat@slonopotamus.org>
Makefile: introduce CROSS_COMPILE variable

To ease cross-compilation process, introduce a single variable
with the prefix to all compiler-related executables.

Define CROSS_COMPILE=foo- if your compiler and binary utilities
are foo-cc, foo-ar, foo-strip, etc.  More specific variables
override this, so if you set CC=gcc CROSS_COMPILE=ia64-linux-gnu-
then the compiler will be 'gcc', not 'ia64-linux-gnu-gcc'.

Signed-off-by: Marat Radchenko <marat@slonopotamus.org>
compat/nedmalloc/malloc.c.h: fix compilation under MinGW-W64

1. Unlike MinGW, MinGW-W64 already provides _ReadWriteBarrier macro,
   so don't try to redefine it.

2. MinGW-W64 has a strange definition FORCEINLINE as
   extern __inline__ __attribute__((__always_inline__,__gnu_inline__))

   'extern' doesn't work together with 'static', so #undef MinGW-W64
   version of FORCEINLINE.

Signed-off-by: Marat Radchenko <marat@slonopotamus.org>
MINGW: config.mak.uname: add explicit way to request MinGW-build

When crosscompiling, one cannot rely on `uname` from host system.
Thus, add an option to use `make MINGW=1` for building MinGW build
from non-MinGW host (Linux, for example).

Signed-off-by: Marat Radchenko <marat@slonopotamus.org>
MINGW: compat/poll/poll.c: undef NOGDI

On MinGW-W64, MsgWaitForMultipleObjects is guarded with #ifndef NOGDI.

Removal -DNOGDI=1 from config.mak.uname has an undesirable effect of
bringing in wingdi.h with weird #define ERROR 0 that conflicts with
internal Git enums. So, just #undef NOGDI in compat/poll/poll.c.

Signed-off-by: Marat Radchenko <marat@slonopotamus.org>