You always build in “Debug” or “Release” (or “DebWithRelInfo” or “MinSizeRel”) configuration. If you don’t specify build mode then I think the default is “Debug” (more verbose debug information but slower execution time).
Your CPU is quite old but it should still be able to build and run Slicer, so it is not a hardware problem.
I did a quick search. There used to be such errors at startup but they have all been fixed. Are you using the latest master version? What is your linux kernel, Qt, OpenSSL, and gcc compiler versions? (you may find some version information in your build log of top-level CMakeCache.txt file)