build system: do not separate the debugging symbols by default

the reason is that the tools for profiling and for coverage analysis
(i.e., gprof and gcov) have trouble with this and crash. if you want
this to be enabled for whatever reason, use the cmake flag
-DSTRIP_DEBUGGING_SYMBOLS=ON or pass configure the
--enable-strip-debug option.
That's not what I'm referring to.  You assign the `-DSTRIP_*` string to the variable `use_lto` when you (probably) intended to assign it to the new shell variable `strip_debug`.