Maxscale 2.5 build fails for SLES12

It also fails on Deb Jessie 8, but since LTS ends June 30th 2020, the MaxScale team is not going to release MaxScale 2.5 for Debian Jessie.*

[ 12%] Built target jansson
Scanning dependencies of target cdc_connector_static
[ 12%] Building CXX object connectors/cdc-connector/CMakeFiles/cdc_connector_static.dir/cdc_connector.cpp.o
c+: error: unrecognized command line option ‘-std=c+14’
c+: error: unrecognized command line option ‘-std=c+14’
connectors/cdc-connector/CMakeFiles/cdc_connector_static.dir/build.make:62: recipe for target 'connectors/cdc-connector/CMakeFiles/cdc_connector_static.dir/cdc_connector.cpp.o' failed
CMakeFiles/Makefile2:6978: recipe for target 'connectors/cdc-connector/CMakeFiles/cdc_connector_static.dir/all' failed
make[2]: *** [connectors/cdc-connector/CMakeFiles/cdc_connector_static.dir/cdc_connector.cpp.o] Error 1
make[1]: *** [connectors/cdc-connector/CMakeFiles/cdc_connector_static.dir/all] Error 2
make[1]: *** Waiting for unfinished jobs....
Scanning dependencies of target maxbase
[ 12%] Building CXX object maxutils/maxbase/src/CMakeFiles/maxbase.dir/alloc.cc.o
[ 13%] Building CXX object maxutils/maxbase/src/CMakeFiles/maxbase.dir/atomic.cc.o
c+: error: unrecognized command line option ‘-std=c+14’
c+: error: unrecognized command line option ‘-std=c+14’
c+: error: unrecognized command line option ‘-std=c+14’
c+: error: unrecognized command line option ‘-std=c+14’
maxutils/maxbase/src/CMakeFiles/maxbase.dir/build.make:62: recipe for target 'maxutils/maxbase/src/CMakeFiles/maxbase.dir/alloc.cc.o' failed
make[2]: *** [maxutils/maxbase/src/CMakeFiles/maxbase.dir/alloc.cc.o] Error 1
make[2]: *** Waiting for unfinished jobs....
maxutils/maxbase/src/CMakeFiles/maxbase.dir/build.make:86: recipe for target 'maxutils/maxbase/src/CMakeFiles/maxbase.dir/atomic.cc.o' failed
make[2]: *** [maxutils/maxbase/src/CMakeFiles/maxbase.dir/atomic.cc.o] Error 1
CMakeFiles/Makefile2:498: recipe for target 'maxutils/maxbase/src/CMakeFiles/maxbase.dir/all' failed
make[1]: *** [maxutils/maxbase/src/CMakeFiles/maxbase.dir/all] Error 2