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, 10 insertions, 3 deletions

diff --git a/Makefile b/Makefile
index 808a9beac..e46eb4949 100644
--- a/Makefile
+++ b/Makefile
@@ -2,6 +2,13 @@
 ROOT = .
 -include config.mk
 
+# Default programs
+CC ?= cc
+CODESPELL ?= codespell
+CPPCHECK ?= cppcheck
+GAWK ?= gawk
+SCAN_BUILD ?= scan-build
+
 ifneq ($(HAVE_MAN),no)
 MAN_TARGET = man
 endif
@@ -357,18 +364,18 @@ extras: all
 
 .PHONY: cppcheck
 cppcheck: clean
-        cppcheck --force --error-exitcode=1 --enable=warning,performance .
+        $(CPPCHECK) --force --error-exitcode=1 --enable=warning,performance .
 
 .PHONY: scan-build
 scan-build: clean
-        scan-build $(MAKE)
+        $(SCAN_BUILD) $(MAKE)
 
 # TODO: Old codespell versions (such as v2.1.0 in CI) have issues with
 # contrib/syscalls.sh
 .PHONY: codespell
 codespell:
         @printf 'Running %s...\n' $@
-        @codespell --ignore-regex 'Manuel|UE|als|chage|creat|doas|ether|isplay|readby|[Ss]hotcut' \
+        @$(CODESPELL) --ignore-regex 'Manuel|UE|als|chage|creat|doas|ether|isplay|readby|[Ss]hotcut' \
           -S *.d,*.gz,*.o,*.so \
           -S COPYING,m4 \
           -S ./contrib/syscalls.sh \