cmake: create compile_commands.json by default

cmake: create compile_commands.json by default

Some users have expressed interest in a more "batteries included" way of
building via CMake[1], and a big part of that is providing easier access
to tooling external tools.

A straightforward way to accomplish this is to make it as simple as
possible is to enable the generation of the compile_commands.json file,
which is supported by many tools such as: clang-tidy, clang-format,
sourcetrail, etc.

This does come with a small run-time overhead during the configuration
step (~6 seconds on my machine):

    Time to configure with CMAKE_EXPORT_COMPILE_COMMANDS=TRUE

    real    1m9.840s
    user    0m0.031s
    sys     0m0.031s

    Time to configure with CMAKE_EXPORT_COMPILE_COMMANDS=FALSE

    real    1m3.195s
    user    0m0.015s
    sys     0m0.015s

This seems like a small enough price to pay to make the project more
accessible to newer users.  Additionally there are other large projects
like llvm [2] which has had this enabled by default for >6 years at the
time of this writing, and no real negative consequences that I can find
with my search-skills.

NOTE: That the compile_commands.json is currently produced only when
using the Ninja and Makefile generators.  See The CMake documentation[3]
for more info.

1: https://lore.kernel.org/git/CAOjrSZusMSvs7AS-ZDsV8aQUgsF2ZA754vSDjgFKMRgi_oZAWw@mail.gmail.com/
2: https://github.com/llvm/llvm-project/commit/2c5712051b31b316a9fc972f692579bd8efa6e67
3: https://cmake.org/cmake/help/latest/variable/CMAKE_EXPORT_COMPILE_COMMANDS.html

Signed-off-by: Matthew Rogers <mattr94@gmail.com>
Acked-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>