cxx/inlib: add simple Makefile

add a simple `Makefile` to build and run the benchmarks

e.g.:
```sh
$ make
g++ -O3 -I. hcontest.cpp -o cxx-hcontest
./cxx-hcontest
h1d.fill(10000000) 2 secs 12731 micro_secs
h2d.fill(10000000) 3 secs 934001 micro_secs
sh1d.fill(10000000) 2 secs 695909 micro_secs
sh2d.fill(10000000) 5 secs 726247 micro_secs
```