libreoffice-{fresh,still}: upgrade and clean

* libreoffice-still: -> 6.0.6.2
  * (newer than our current 'fresh!')
* libreoffice-fresh: -> 6.1.0.3
  * 6.1.1(.1) is currently pre-release, FWIW
* Use normal gcc, not gcc5
  * dropping 'glibc' from buildInputs fixed this (?)
* remove many fixes/touchups/workarounds/hacks
  * hopefully everything still works for everyone
* disable online update since that seems unlikely to work anyway
* fix autogen/configure invocations
* disable libnumbertext in 6.1.x since not packaged
* drop 'touch solenv/inc/target.mk' as unclear what it was for
  and doesn't seem to be currently needed
* cleanup link gen a bit[1]
* split checks to check phase

[1]
primary motivation was to stop creating links like:

'libreoffice-6.0.5.2/src/-libxslt-1.1.32.tar.gz' -> '/nix/store/503v5hmhm430bld0h078gacmkniwdllr-libxslt-1.1.32.tar.gz'
'libreoffice-6.0.5.2/src/libxslt-1.1.32.tar.gz' -> '/nix/store/503v5hmhm430bld0h078gacmkniwdllr-libxslt-1.1.32.tar.gz'

This is mostly accomplished by simply using the 'md5name' field
which the python script kindly generates for us
(including the use of non-md5 if md5 is not set or empty).