build: use -pthread when available

Our code currently tries to link -lpthread and adds stuff like -D_REENTRANT
based on the target platform.

GCC actually supports to just pass a -pthread compiler and linker flag that
will automatically enable threading and define the correct symbols for the
platform, so let's try to just use that as our first choice.

clang also supports -pthread but it must be used only as a compiler flag,
so we also take care of that scenario with this commit.