Silence warning about "using serial compilation"

On Ubuntu, one sees warnings like this:

```
lto-wrapper: warning: using serial compilation of 13 LTRANS jobs
lto-wrapper: note: see the ‘-flto’ option documentation for more
information
lto-wrapper: warning: using serial compilation of 15 LTRANS jobs
lto-wrapper: note: see the ‘-flto’ option documentation for more
information
lto-wrapper: warning: using serial compilation of 16 LTRANS jobs
lto-wrapper: note: see the ‘-flto’ option documentation for more
information
lto-wrapper: warning: using serial compilation of 16 LTRANS jobs
lto-wrapper: note: see the ‘-flto’ option documentation for more
information
```

This seems to be the default behavior if the option `-flto` is not
given a value (c.f. https://stackoverflow.com/a/72222512/28972686).
Giving it a value of "auto" makes the warning go away and lets the
compilation toolchain decide how much parallism it can use.

Author: mhucka

Makefile

@@ -36,7 +36,7 @@ CXX ?= g++
 NVCC ?= nvcc
 HIPCC ?= hipcc
 
-CXXFLAGS ?= -O3 -std=c++17 -fopenmp
+CXXFLAGS ?= -O3 -std=c++17 -fopenmp -flto=auto
 NVCCFLAGS ?= -O3 --std c++17 -Wno-deprecated-gpu-targets
 HIPCCFLAGS ?= -O3