Fix warnings in nedmalloc when compiling with GCC 4.4.0

Fix warnings in nedmalloc when compiling with GCC 4.4.0

Nedmalloc's source code has a cute #define construct to avoid inserting
an if() statement, because that might interact badly with enclosing if()
statements.  However, GCC > 4 complains with a "warning: value computed
is not used".  So we cast the result to "void".

GCC also does not understand the Visual C++ specific pragmas, so we need
to disable them for MinGW.

We need to include malloc.h on Windows even if we happen to compile the
stuff as a MinGW program.  Otherwise the function declaration of alloca()
is missing.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>