build: allow overriding certain tools

Allow overriding the following tools at configure-time and build-time:

* codespell
* cppcheck
* gawk
* scan-build

For example, instead of hardcoding `gawk`, enable overriding it at
configure-time with:

    ./configure GAWK=/path/to/gawk

To override it for a single `make` invocation:

    make GAWK=/path/to/gawk

Also, add default values for the programs that are not found (rather
than leaving the variables empty), to make error messages clearer when
trying to run them:

    $ make CPPCHECK= cppcheck-old
    [...]
    force --error-exitcode=1 --enable=warning,performance .
    make: force: No such file or directory
    $ make CPPCHECK=cppcheck cppcheck-old
    [...]
    cppcheck --force --error-exitcode=1 --enable=warning,performance .
    make: cppcheck: No such file or directory

0 files changed, 0 insertions, 0 deletions