Let hipcc link -lpthread -lm by default

[yxsamliu]

rocRAND assumes hipcc linking with -lpthread -lm by default.

[aaronenyeshi]

Thanks Sam, does this fix this PR ROCm/rocRAND#29

[yxsamliu]

yes

[ax3l]

This patch is very unstable and does not work with modern CMake, which directly uses clang -x hip.
rocRAND (and all other ROC/HIP packages) should publicly declare their dependencies instead of hoping that the hipcc wrapper is used to fix missing declarations of dependencies.

X-ref: ROCm/rocRAND#29 (comment)