Added check for : command not producing .gmo file in po/Makefile.in.in

[domainexpert]

Otherwise the make may fail with the following message when msgfmt is not installed:

make[2]: Entering directory '/root/w3m/po'
rm -f ja.gmo && : -c --statistics --verbose -o ja.gmo ja.po
mv: cannot stat 't-ja.gmo': No such file or directory
Makefile:132: recipe for target 'ja.gmo' failed
make[2]: *** [ja.gmo] Error 1
make[2]: Leaving directory '/root/w3m/po'
Makefile:169: recipe for target 'stamp-po' failed
make[1]: *** [stamp-po] Error 2
make[1]: Leaving directory '/root/w3m/po'
Makefile:213: recipe for target 'po' failed
make: *** [po] Error 2

[tats]

Merged, thank you.