building libvfio-user generates output even if nothing is built

[tmakatos]

CMake generates the following output when nothing needs building:

# make
[  4%] Built target libvfio-user_ut
[  8%] Built target pci_caps_ut
[ 11%] Built target pci_caps
[ 13%] Built target irq
[ 15%] Built target dma
[ 17%] Built target tran_sock
[ 22%] Built target dma_ut
[ 24%] Built target pci
[ 28%] Built target tran_sock_ut
[ 31%] Built target migration
[ 33%] Built target libvfio-user
[ 35%] Built target vfio-user-static
[ 37%] Built target vfio-user-shared
[ 42%] Built target irq_ut
[ 46%] Built target pci_ut
[ 51%] Built target migration_ut
[ 55%] Built target server
[ 60%] Built target gpio-pci-idio-16
[ 68%] Built target client
[ 73%] Built target lspci
[ 77%] Built target null
[100%] Built target unit-tests

Ideally we should be able to hide this output because it creates unnecessary noise when used in larger projects., Maybe make it configurable?

[jlevon]

If you can figure out how to turn it off, please do so!

[tmakatos]

It's suprisingly non-trivial (?!): https://stackoverflow.com/questions/34277015/avoid-the-progress-messages-in-cmake