FIX: changed find strategy of cmake for python

[HenningScheufler]

For the linux system, libpythonfmu-export.so has to be unique for every python version and readelf -d libpythonfmu-export.so should link to the corresponding python version (here: 3.7 )

0x0000000000000001 (NEEDED) Gemeinsame Bibliothek [libpython3.7m.so.1.0] 0x0000000000000001 (NEEDED) Gemeinsame Bibliothek [libstdc++.so.6] 0x0000000000000001 (NEEDED) Gemeinsame Bibliothek [libgcc_s.so.1] 0x0000000000000001 (NEEDED) Gemeinsame Bibliothek [libc.so.6] 0x000000000000000e (SONAME) soname der Bibliothek: [libpythonfmu-export.so]

NOTE: this fix requires CMAKE 3.15 ubuntu 20.04 ships with 3.16

[markaren]

Perhaps better to build with ubuntu-18.04 and update cmake (pip install cmake) ? The real question is if ubuntu-18.04 users can use the binary built by a newer version?