Linux-5.17: Kernel build uses -Wcast-function-type

The linux 5.17 commit:
  "Makefile: Enable -Wcast-function-type" (552a23a0)
added the -Wcast-function-type compiler flag for kernel module builds.

This change catches a type mismatch in the external files obtained from
heimdal: hcrypto/evp.c and hcrypto/evp-algs.c and produces the following
type of compile time error messages.

  src/libafs/MODLOAD-.../evp.c: In function ‘hc_EVP_md_null’:
  src/libafs/MODLOAD-.../evp.c:501:2: error: cast between incompatible
      function types from ‘void (*)(void *)’ to ‘int (*)(EVP_MD_CTX *)’
          {aka ‘int (*)(struct hc_EVP_MD_CTX *)’}
          [-Werror=cast-function-type]
  501 |  (hc_evp_md_init)null_Init,
      |  ^

Use AX_APPEND_COMPILE_FLAGS to create a CFLAGS_NOCAST_FUNCTION_TYPE
macro to disable this warning and update the CFLAGS for these 2 files
for the Linux libafs build.

Update the CODING documentation to add the new exceptions.  In addition
add a brief description on how to set up autoconf to add a new build
macro to suppress compiler warnings.

Note: upstream heimdal has committed a fix for this in:

   hcrypto: Fix return type for null_Init, null_Update and null_Final
   (fc4b3ce49b)

Change-Id: I05b716867016a33ca02a791ed6bc5a7d846de608
Reviewed-on: https://gerrit.openafs.org/14881
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>

CODING

@@ -306,7 +306,13 @@ the fix is to mark that warning as ignored, but only for clang. For example:
   # endif
   #endif
 
-If a pragma isn't available for your particular warning, you will need to
+If the source cannot be changed to add a pragma, you might be abe to use the
+autoconf function AX_APPEND_COMPILE_FLAGS to create a new macro that disables
+the warning and then use macro for the build options for that file. For an
+example, see how the autoconf macro CFLAGS_NOIMPLICIT_FALLTHROUGH is defined and
+used.
+
+Finally if there isn't a way to disable the specific warning, you will need to
 disable all warnings for the file in question. You can do this by supplying
 the autoconf macro @CFLAGS_NOERROR@ in the build options for the file. For
 example:
@@ -332,6 +338,10 @@ bucoord/commands.c   : all	     : Ubik_Call
 				     : signed vs unsigned for dates
 butc/tcudbprocs.c    : all	     : ubik_Call
 external/heimdal/hcrypto/validate.c: all: statement with empty body
+external/heimdal/hcrypto/evp.c:      cast-function-type
+             : Linux kernel build uses -Wcast-function-type
+external/heimdal/hcrypto/evp-algs.c: cast-function-type
+             : Linux kernel build uses -Wcast-function-type
 kauth/admin_tools.c  : strict-proto  : ubik_Call
 kauth/authclient.c   : strict-proto  : ubik_Call nonsense
 libadmin/kas/afs_kasAdmin.c: strict-proto : ubik_Call nonsense

src/cf/osconf.m4

@@ -630,6 +630,7 @@ CFLAGS_NOSTRICT=-fno-strict-aliasing
 CFLAGS_NOUNUSED=
 CFLAGS_NOOLDSTYLE=
 CFLAGS_NOIMPLICIT_FALLTHROUGH=
+CFLAGS_NOCAST_FUNCTION_TYPE=
 XCFLAGS_NOCHECKING="$XCFLAGS"
 
 if test "x$GCC" = "xyes"; then
@@ -645,6 +646,8 @@ if test "x$GCC" = "xyes"; then
       CFLAGS_NOOLDSTYLE="-Wno-old-style-definition"
       AX_APPEND_COMPILE_FLAGS([-Wno-implicit-fallthrough],
                                [CFLAGS_NOIMPLICIT_FALLTHROUGH])
+      AX_APPEND_COMPILE_FLAGS([-Wno-cast-function-type],
+                              [CFLAGS_NOCAST_FUNCTION_TYPE])
       AC_DEFINE(IGNORE_SOME_GCC_WARNINGS, 1, [define to disable some gcc warnings in warnings-as-errors mode])
     else
       CFLAGS_NOSTRICT=
@@ -728,6 +731,7 @@ AC_SUBST(CFLAGS_NOSTRICT)
 AC_SUBST(CFLAGS_NOUNUSED)
 AC_SUBST(CFLAGS_NOOLDSTYLE)
 AC_SUBST(CFLAGS_NOIMPLICIT_FALLTHROUGH)
+AC_SUBST(CFLAGS_NOCAST_FUNCTION_TYPE)
 AC_SUBST(CFLAGS_WERROR)
 AC_SUBST(XCFLAGS64)
 AC_SUBST(XLDFLAGS)

src/libafs/MakefileProto.LINUX.in

@@ -79,8 +79,9 @@ CFLAGS_rxkad_common.o = -I${TOP_SRCDIR}/rxkad -I$(TOP_OBJDIR)/src/rxkad
 CFLAGS_opr_rbtree.o = -I${TOP_SRCDIR}/opr
 
 CFLAGS_evp.o = -I$(TOP_SRCDIR)/external/heimdal/hcrypto \
-	       -DHAVE_CONFIG_H
+	       -DHAVE_CONFIG_H @CFLAGS_NOCAST_FUNCTION_TYPE@
-CFLAGS_evp-algs.o = -I$(TOP_SRCDIR)/external/heimdal/hcrypto
+CFLAGS_evp-algs.o = -I$(TOP_SRCDIR)/external/heimdal/hcrypto \
+	       @CFLAGS_NOCAST_FUNCTION_TYPE@
 CFLAGS_evp-kernel.o = -I$(TOP_SRCDIR)/external/heimdal/hcrypto
 CFLAGS_rand-timer-kernel.o = -I$(TOP_SRCDIR)/external/heimdal/hcrypto
 CFLAGS_rand-kernel.o = -I$(TOP_SRCDIR)/external/heimdal/hcrypto