aclocal.m4: call cygpath on mingw32 only

Authored by trofi on Sep 20 2017, 5:18 PM.

Description

aclocal.m4: call cygpath on mingw32 only

The only reason I noticed is warning these lines on linux:

$ ./configure --target=sparc-unknown-linux-gnu
...
./configure: line 9708: cygpath: command not found
./configure: line 9708: ArCmd: command not found

POSIX shell syntax requires no spaces in assignments.
Fixed guarding condition while at it.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>