After some modifications are applied, I've got:

...
configure:3331: checking for cross-compile version of gcc
configure:3387: result: no
configure:3423: checking for Windows native path bug in windres
configure:3427: windres -o conftest.res.o C:\cygwin64\tmp\conftest.rc
gcc: error: C:cygwin64tmpconftest.rc: No such file or directory
gcc: warning: '-x c' after last input file has no effect
gcc: fatal error: no input files
compilation terminated.
windres: preprocessing failed.
configure:3430: $? = 1
configure:3435: result: yes
configure:3452: checking for mingw32 version of gcc
configure:3477: gcc -c -O -I/usr/pkg/include -I/usr/pkg/include conftest.c >&5
configure:3483: $? = 0
configure:3487: test -z 
                         || test ! -s conftest.err
configure:3490: $? = 0
configure:3493: test -s conftest.o
configure:3496: $? = 0
configure:3508: result: no
configure:3511: error: gcc cannot produce win32 executables.

Seems backslashes are stripped out. No, it is the function of windres ;-(