graphics/pngcrush: Fix x86 build.

Signed-off-by: B. Watson <yalhcru@gmail.com>

graphics/pngcrush/pngcrush.SlackBuild

@@ -23,6 +23,7 @@
 #  ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 # Modified by Eugene M., <damagedone at gmx dot com>
+# Modified by B. Watson <yalhcru@gmail.com>, fix x86 build error
 
 PRGNAM=pngcrush
 VERSION=${VERSION:-1.8.11}
@@ -57,6 +58,10 @@ find -L . \
  \( -perm 666 -o -perm 664 -o -perm 640 -o -perm 600 -o -perm 444 \
  -o -perm 440 -o -perm 400 \) -exec chmod 644 {} \;
 
+# 20170603 bkw: this appears to be the right way to fix bug #75:
+# https://sourceforge.net/p/pmt/bugs/75/
+sed -i '/typedef long ptrdiff_t;/s,long,__PTRDIFF_TYPE__,' zutil.h
+
 make
 install -D -m 0755 pngcrush $PKG/usr/bin/pngcrush