Disable STRIP operations when appropriate.

[RonxBulld]

What are the reasons/motivation for this change?
It was always difficult to debug the logic in yosys when using it as a dynamic library, and it was further discovered that this was because STRIP was always used to remove symbol information in yosys when installing yosys, regardless of how ENABLE_DEBUG was set. I don't know if this is by design or just an unexpected behavior.

Explain how this is achieved.
The STRIP variable is cleared when ENABLE_DEBUG is set to 1, and validity checks are performed everywhere the STRIP variable is used.

If applicable, please suggest to reviewers how they can test the change.
Build with ENABLE_DEBUG=1 and inspect with a debug symbol viewing tool.

[widlarizer]

The core of the issue is that make install strips installed yosys and libyosys.so, regardless of ENABLE_DEBUG. This seems wrong, and this PR is a valid solution to this

I see a potential problem with somebody setting a custom STRIP variable regardless of if they set ENABLE_DEBUG, but also, I'm willing to bet that no such person exists, so I don't mind.

Also, beware: strip -S creates binaries that file reports as not stripped... terrifying

[RonxBulld]

The core of the issue is that make install strips installed yosys and libyosys.so, regardless of ENABLE_DEBUG. This seems wrong, and this PR is a valid solution to this

I see a potential problem with somebody setting a custom STRIP variable regardless of if they set ENABLE_DEBUG, but also, I'm willing to bet, so I don't mind.

Also, beware: strip -S creates binaries that file reports as not stripped... terrifying

You summed it up more clearly than I could.
Yes, I also noticed the "not stripped" problem you mentioned. The file tool does not seem to think that ELF without DWARF is "stripped".
For the other question, if the user executes "make" and simply leaves STRIP empty in the hope that STRIP will not be executed, it will cause another problem: the shell tries to execute a command called "-S".

[widlarizer]

I thought of an evil alternative, by the way: if STRIP is set to true, the true shell builtin will be called, which does nothing and exits with 0. So stripping may be disabled by setting STRIP=true - by the user, or automatically when ENABLE_DEBUG=1. This is a neat solution but very confusing as well