integration fix: change the way to print the version

Using "system" to call "git log" is too error-prone.
In the test setup, lte-softmodem was blocked because
"git log" uses a pager ("less") and the pager was
waiting for the user to press enter.

We could pass "--no-pager" to "git log" but then
what if someone runs the softmodem out of the source
tree?

cmake defines PACKAGE_VERSION, we can use that instead,
as is done in openair-cn.