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

1 files changed, 5 insertions, 0 deletions

diff --git a/config.mk.in b/config.mk.in
index 1521a2633..253b86435 100644
--- a/config.mk.in
+++ b/config.mk.in
@@ -75,6 +75,11 @@ MANFLAGS = \
 # the user building the package); see the following for details:
 # https://www.gnu.org/software/automake/manual/1.16.5/html_node/User-Variables.html
 CC=@CC@
+CODESPELL=@CODESPELL@
+CPPCHECK=@CPPCHECK@
+GAWK=@GAWK@
+SCAN_BUILD=@SCAN_BUILD@
+
 CFLAGS=@CFLAGS@
 CPPFLAGS=@CPPFLAGS@
 LDFLAGS=@LDFLAGS@