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, 2 deletions

diff --git a/configure.ac b/configure.ac
index 7a2c379fe..2762c2ef0 100644
--- a/configure.ac
+++ b/configure.ac
@@ -19,6 +19,10 @@ AC_CONFIG_SRCDIR([src/firejail/main.c])
 AC_CONFIG_MACRO_DIR([m4])
 
 AC_PROG_CC
+AC_CHECK_PROGS([CODESPELL], [codespell])
+AC_CHECK_PROGS([CPPCHECK], [cppcheck])
+AC_CHECK_PROGS([GAWK], [gawk])
+AC_CHECK_PROGS([SCAN_BUILD], [scan-build])
 
 DEPS_CFLAGS=""
 AC_SUBST([DEPS_CFLAGS])
@@ -139,8 +143,7 @@ AC_ARG_ENABLE([man],
     [AS_HELP_STRING([--disable-man], [disable man pages])])
 AS_IF([test "x$enable_man" != "xno"], [
         HAVE_MAN="-DHAVE_MAN"
-        AC_CHECK_PROG([HAVE_GAWK], [gawk], [yes], [no])
-        AS_IF([test "x$HAVE_GAWK" != "xyes"], [AC_MSG_ERROR([*** gawk not found ***])])
+        AS_IF([test "x$GAWK" = "x"], [AC_MSG_ERROR([*** gawk not found ***])])
 ])
 
 HAVE_PRIVATE_HOME=""