Corrected the case of the install source variable

This was causing a problem for make install as JsonCpp was needed (not
JSONCPP) to match the project name.

Change-Id: Ib57123603af0769450d3e3902ae0f1d5a8ef30b3
Merge topic 'jsoncpp-system-option' into master

874a5bb5 Corrected the case of the install source variable