RFR: JDK-8217448 Check for pandoc availability using ENABLE_PANDOC and not PANDOC

[Magnus Ihse Bursie]

In some places, we have checked if we have pandoc available by comparing 
$(PANDOC) with the empty string, assuming that non-empty means that 
we've found pandoc.

However, on Windows, PANDOC will always contain $(FIXPATH), so this is 
not a valid test. Instead we should use ENABLE_PANDOC.

Bug: https://bugs.openjdk.java.net/browse/JDK-8217448
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8217448-fix-pandoc-on-windows/webrev.01

/Magnus