Fix:

error: implicitly declaring library function 'exit' with type 'void (int) __attribute__((noreturn))' [-Werror,-Wimplicit-function-declaration]

error: implicitly declaring library function 'strcmp' with type 'int (const char *, const char *)' [-Werror,-Wimplicit-function-declaration]
--- configure.orig	2020-12-20 06:03:47.000000000 -0600
+++ configure	2020-12-20 06:07:49.000000000 -0600
@@ -238,6 +238,7 @@
 
 echo checking for sys/termios.h
 echo "#include <sys/termios.h>
+#include <stdlib.h>
 main() { exit(0); }" > conftest.c
 eval $compile
 if test -s conftest && ./conftest 2>/dev/null; then
@@ -247,6 +248,7 @@
 
 echo checking for POSIX.1 header files
 echo "#include <unistd.h>
+#include <stdlib.h>
 main() {
 #ifdef _POSIX_VERSION
 exit(0);
@@ -262,6 +264,7 @@
 
 echo checking for BSD string and memory functions
 echo "#include <strings.h>
+#include <stdlib.h>
 main() { exit(0); rindex(0, 0); bzero(0, 0); }" > conftest.c
 eval $compile
 if test -s conftest && ./conftest 2>/dev/null; then :
@@ -271,6 +274,7 @@
 
 echo checking whether sys/types.h defines uid_t
 echo '#include <sys/types.h>
+#include <stdlib.h>
 main() { uid_t x; exit(0); }' > conftest.c
 eval $compile
 if test -s conftest && ./conftest 2>/dev/null; then :
@@ -291,6 +295,7 @@
 
 echo checking how to get alloca
 echo '
+#include <stdlib.h>
 #ifdef __GNUC__
 #define alloca __builtin_alloca
 #else
@@ -316,6 +321,7 @@
 if [ $usenls = true ]; then
   echo checking for nls
   echo '#include <nl_types.h>
+  #include <stdlib.h>
   main() {nl_catd catfd; exit(0); }' > conftest.c
   eval $compile
   if test -s conftest && ./conftest 2>/dev/null; then :
@@ -333,6 +339,7 @@
 echo '#define _GNU_SOURCE
 #include <getopt.h>
 #include <stdio.h>
+#include <stdlib.h>
 struct option long_opts[] = { { "", no_argument, NULL, 0 } };
 main() { exit(0); }' > conftest.c
 eval $compile
--- src/makemsg.c.orig	2006-08-02 13:11:44.000000000 -0500
+++ src/makemsg.c	2020-11-14 09:51:34.000000000 -0600
@@ -19,6 +19,7 @@
  */
 #include <stdio.h>
 #include <stdlib.h>
+#include <string.h>
 #include <unistd.h>
 #ifdef __QNX__
 #include <unix.h>