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, 1 insertions, 1 deletions

diff --git a/src/zsh_completion/Makefile b/src/zsh_completion/Makefile
index bf83877fb..e964d39ec 100644
--- a/src/zsh_completion/Makefile
+++ b/src/zsh_completion/Makefile
@@ -6,7 +6,7 @@ ROOT = ../..
 all: _firejail
 
 _firejail: _firejail.in $(ROOT)/config.mk
-        gawk -f ../man/preproc.awk -- $(MANFLAGS) < $< > $@.tmp
+        $(GAWK) -f ../man/preproc.awk -- $(MANFLAGS) < $< > $@.tmp
         sed "s|_SYSCONFDIR_|$(sysconfdir)|" < $@.tmp > $@
         rm $@.tmp