Temporarily remove PCRE from the benchmarking system.

This prepares a restructuration of the benchmarking system.  PCRE currently
does not build on my machine, so this makes it easier.
Temporarily remove v8 from the benchmarks.

This prepares for the restructuration of the benchmarking system. The v8
benchmarks will be reintroduced in a later commit.
Let scripts check that executables are installed to avoid unclear error messages.
Extract and share code for benchmarking engines.
Benchmark engines now only benchmark finding all matches in the text.
Benchmark engines can benchmark multiple text sizes.

Benchmark engines now initialise one text for the maximum size required, and
use pieces of the text for smaller text sizes.
Let the benchmark engines output the results nicely indented.
Remove unused benchmarking python file.
Update benchmarking scripts to pass a list of sizes to engines.
Add more arguments to the benchmarks script.
Remove unused python code in the benchmark scripts.