Avoid implicit ints and implicit function declarations during the
(non-autoconf) build environment probing stage.

This prevents build failures with future compilers.

diff -ur man-1.6g.orig/configure man-1.6g/configure
--- man-1.6g.orig/configure	2010-12-31 21:28:46.000000000 +0100
+++ man-1.6g/configure	2023-02-07 19:08:27.312390739 +0100
@@ -223,7 +223,7 @@
 echo checking for ANSI C header files
 echo "#include <stdlib.h>
 #include <string.h>
-main() { exit(0); strerror(0); }" > conftest.c
+int main(void) { exit(0); strerror(0); }" > conftest.c
 eval $compile
 if test -s conftest && ./conftest 2>/dev/null; then
   DEFS="$DEFS -DSTDC_HEADERS"
@@ -236,7 +236,7 @@
 
 echo checking for sys/termios.h
 echo "#include <sys/termios.h>
-main() { exit(0); }" > conftest.c
+int main(void) { return 0; }" > conftest.c
 eval $compile
 if test -s conftest && ./conftest 2>/dev/null; then
   DEFS="$DEFS -DTERMIOS_HEADER"
@@ -245,11 +245,11 @@
 
 echo checking for POSIX.1 header files
 echo "#include <unistd.h>
-main() {
+int main(void) {
 #ifdef _POSIX_VERSION
-exit(0);
+return 0;
 #else
-exit(1);
+return 1;
 #endif
 }" > conftest.c
 eval $compile
@@ -260,7 +260,7 @@
 
 echo checking for BSD string and memory functions
 echo "#include <strings.h>
-main() { exit(0); rindex(0, 0); bzero(0, 0); }" > conftest.c
+int main(void) { return 0; rindex(0, 0); bzero(0, 0); }" > conftest.c
 eval $compile
 if test -s conftest && ./conftest 2>/dev/null; then :
   else DEFS="$DEFS -DUSG"
@@ -269,7 +269,7 @@
 
 echo checking whether sys/types.h defines uid_t
 echo '#include <sys/types.h>
-main() { uid_t x; exit(0); }' > conftest.c
+int main(void) { uid_t x; return 0; }' > conftest.c
 eval $compile
 if test -s conftest && ./conftest 2>/dev/null; then :
 else
@@ -302,7 +302,7 @@
 #endif
 #endif
 #endif
-main() { char *p = (char *) alloca(1); exit(0); }' > conftest.c
+int main(void) { char *p = (char *) alloca(1); return 0; }' > conftest.c
 eval $compile
 if test -s conftest && ./conftest 2>/dev/null; then :
 elif test -d /usr/ucblib; then LIBS="$LIBS -L/usr/ucblib -lucb"
@@ -314,7 +314,7 @@
 if [ $usenls = true ]; then
   echo checking for nls
   echo '#include <nl_types.h>
-  main() {nl_catd catfd; exit(0); }' > conftest.c
+  int main(void) {nl_catd catfd; return 0; }' > conftest.c
   eval $compile
   if test -s conftest && ./conftest 2>/dev/null; then :
   else
@@ -332,7 +332,7 @@
 #include <getopt.h>
 #include <stdio.h>
 struct option long_opts[] = { { "", no_argument, NULL, 0 } };
-main() { exit(0); }' > conftest.c
+int main(void) { return 0; }' > conftest.c
 eval $compile
 if test -s conftest && ./conftest 2>/dev/null; then
   manpathoption="--path"