This is follow-up patch to the review https://reviews.llvm.org/D100185.
gdb-plugin code is a interface between gdb, libompd, gdb and libomp. and Python3 development files are required to build this utility.
This patch will disable the plugin code build when python3 dev files are not available.
I think option() is better for booleans.
https://cmake.org/cmake/help/latest/command/option.html