fix ISBLANK() for Visual Studio < 2013 (ref #2658)

Visual Studio versions older than 2013 lack C99's isblank().
Since only ASCII characters are passed to it, implement it
directly without calling the locale-specific isblank().
Merge pull request #2660 from cremno/fix-isblank-for-visual-studio-versions-older-than-2013

fix ISBLANK() for Visual Studio < 2013