Unix|qmake: Added build option for using ccache

“deng_ccache” now makes the build use ccache for faster compilation.
Merge branch 'master' into oculus-rift
UI|Client: Added shadow behind ProgressWidget’s text

Make it more readable on light backgrounds.
libdeng2|Log: Use (v) to identify verbose messages
Unix|qmake: Using ccache with gcc

The previous options were only suitable for clang.
Merge branch 'oculus-rift' into gl2-lensflare