mingw conflict between mkdtemp and sys_stat

[Eric Blake]

I found that M4 is now failing to compile on mingw ever since I imported the 
clean-temp module due to a conflict between modules:

gcc -mno-cygwin -DHAVE_CONFIG_H -DSIGNAL_SAFE_LIST -I. -I.. -I../../lib     -
g2 -Wall -Werror -MT mkdtemp.o -MD -MP -MF .deps/mkdtemp.Tpo -c -o 
mkdtemp.o ../../lib/mkdtemp.c
../../lib/mkdtemp.c:79:1: "mkdir" redefined
In file included from ../../lib/mkdtemp.c:52:
./sys/stat.h:45:1: this is the location of the previous definition

Any chance we can fix the mkdtemp module depend on the sys_stat module, which 
already takes care of mkdir for mingw, rather than open-coding its own mkdir 
replacement?  And would this be any closer to the glibc implementation?

Also, is there any way to move the -DSIGNAL_SAFE_LIST out of the command line 
and into config.h, to avoid clutter in the make output?

-- 
Eric Blake