https://bugs.gentoo.org/869203

See also OE's variant: https://cgit.openembedded.org/openembedded-core/commit/?id=f898db2607ba3837f81292af92bc8cb605b96cb3
--- a/unix/configure
+++ b/unix/configure
@@ -32,7 +32,7 @@ CFLAGS_BZ=''
 echo 'Check C compiler type (optimization options)'
 # Sun C?
 cat > conftest.c << _EOF_
-int main()
+int main(void)
 {
 #ifndef __SUNPRO_C
    bad code
@@ -47,7 +47,7 @@ if test $? -eq 0; then
 else
   # Tru64 DEC/Compaq/HP C?
   cat > conftest.c << _EOF_
-int main()
+int main(void)
 {
 #ifndef __DECC
    bad code
@@ -62,7 +62,7 @@ _EOF_
   else
     # HP-UX HP C?
     cat > conftest.c << _EOF_
-int main()
+int main(void)
 {
 #ifdef __GNUC__
    bad code
@@ -88,7 +88,7 @@ _EOF_
     else
       # GNU C?
       cat > conftest.c << _EOF_
-int main()
+int main(void)
 {
 #ifndef __GNUC__
    bad code
@@ -197,7 +197,7 @@ else
       echo "  Check if OS already has bzip2 library installed"
       cat > conftest.c << _EOF_
 #include "bzlib.h"
-int main()
+int main(void)
 {
   bz_stream strm;
   BZ2_bzCompressEnd(&strm);
@@ -260,7 +260,7 @@ rm -f _match.s _match.o _crc_i386.s _crc_i386.o
 
 echo Check for ANSI options
 cat > conftest.c << _EOF_
-int main()
+int main(void)
 {
 #ifndef __STDC__
    forget it
@@ -288,7 +288,7 @@ $CC $CFLAGS -c conftest.c > /dev/null 2>/dev/null
 echo Check the handling of const
 cat > conftest.c << _EOF_
 typedef int charset[2];
-int main()
+int main(void)
 {
   const charset x;
   const char *foo;
@@ -303,7 +303,7 @@ echo Check for time_t
 cat > conftest.c << _EOF_
 #include <sys/types.h>
 #include <time.h>
-int main()
+int main(void)
 {
   time_t t;
   return 0;
@@ -315,8 +315,9 @@ $CC $CFLAGS -c conftest.c >/dev/null 2>/dev/null
 
 echo Check for size_t
 cat > conftest.c << _EOF_
+#include <stddef.h>
 #include <sys/types.h>
-int main()
+int main(void)
 {
   size_t s;
   return 0;
@@ -329,7 +330,7 @@ $CC $CFLAGS -c conftest.c >/dev/null 2>/dev/null
 echo Check for off_t
 cat > conftest.c << _EOF_
 #include <sys/types.h>
-int main()
+int main(void)
 {
   off_t s;
   return 0;
@@ -353,7 +354,7 @@ cat > conftest.c << _EOF_
 #include <sys/stat.h>
 #include <unistd.h>
 #include <stdio.h>
-int main()
+int main(void)
 {
   struct stat s;
 
@@ -409,7 +410,7 @@ cat > conftest.c << _EOF_
 #include <sys/stat.h>
 #include <unistd.h>
 #include <stdio.h>
-int main()
+int main(void)
 {
   off_t offset;
   struct stat s;
@@ -453,7 +454,7 @@ echo Check for wide char support
 cat > conftest.c << _EOF_
 #include <stdlib.h>
 #include <stdio.h>
-int main()
+int main(void)
 {
   int wsize;
   wchar_t *wide_string;
@@ -489,7 +490,7 @@ fi
 echo Check for gcc no-builtin flag
 # -fno-builtin since version 2
 cat > conftest.c << _EOF_
-int main()
+int main(void)
 {
 #if __GNUC__ >= 2
    return 0;
@@ -508,14 +509,14 @@ $CC $CFLAGS -c conftest.c >/dev/null 2>/dev/null
 for func in rmdir strchr strrchr rename mktemp mktime mkstemp
 do
   echo Check for $func
-  echo "int main(){ $func(); return 0; }" > conftest.c
+  echo "char $func(); int main(void){ return $func(); }" > conftest.c
   $CC $BFLAG -o conftest conftest.c >/dev/null 2>/dev/null
   [ $? -ne 0 ] && CFLAGS="${CFLAGS} -DNO_`echo $func | tr '[a-z]' '[A-Z]'`"
 done
 
 
 echo Check for memset
-echo "int main(){ char k; memset(&k,0,0); return 0; }" > conftest.c
+echo "int main(void){ char memset(); return memset(); }" > conftest.c
 $CC -o conftest conftest.c >/dev/null 2>/dev/null
 [ $? -ne 0 ] && CFLAGS="${CFLAGS} -DZMEM"
 
@@ -523,7 +524,7 @@ $CC -o conftest conftest.c >/dev/null 2>/dev/null
 echo Check for memmove
 cat > conftest.c << _EOF_
 #include <string.h>
-int main() { int a; int b = 0; memmove( &a, &b, sizeof( a)); return a; }
+int main(void) { int a; int b = 0; memmove( &a, &b, sizeof( a)); return a; }
 _EOF_
 $CC -o conftest conftest.c >/dev/null 2>/dev/null
 [ $? -ne 0 ] && CFLAGS="${CFLAGS} -DNEED_MEMMOVE"
@@ -532,7 +533,7 @@ $CC -o conftest conftest.c >/dev/null 2>/dev/null
 echo Check for strerror
 cat > conftest.c << _EOF_
 #include <string.h>
-int main() { strerror( 0); return 0; }
+int main(void) { strerror(0); return 0; }
 _EOF_
 $CC -o conftest conftest.c >/dev/null 2>/dev/null
 [ $? -ne 0 ] && CFLAGS="${CFLAGS} -DNEED_STRERROR"
@@ -540,7 +541,7 @@ $CC -o conftest conftest.c >/dev/null 2>/dev/null
 echo Check for errno declaration
 cat > conftest.c << _EOF_
 #include <errno.h>
-main()
+int main(void)
 {
   errno = 0;
   return 0;
@@ -552,7 +553,7 @@ $CC $CFLAGS -c conftest.c >/dev/null 2>/dev/null
 
 echo Check for directory libraries
 cat > conftest.c << _EOF_
-int main() { return closedir(opendir(".")); }
+int main(void) { char closedir(); char opendir(); return closedir(opendir()); }
 _EOF_
 
 $CC -o conftest conftest.c >/dev/null 2>/dev/null
@@ -574,7 +575,7 @@ fi
 # Dynix/ptx 1.3 needed this
 
 echo Check for readlink
-echo "int main(){ return readlink(); }" > conftest.c
+echo "int main(void){ char readlink(); return readlink(); }" > conftest.c
 $CC -o conftest conftest.c >/dev/null 2>/dev/null
 if [ $? -ne 0 ]; then
   $CC -o conftest conftest.c -lseq >/dev/null 2>/dev/null
@@ -617,7 +618,7 @@ CFLAGS="${CFLAGS} ${OPT}"
 
 echo Check for valloc
 cat > conftest.c << _EOF_
-main()
+int main(void)
 {
 #ifdef MMAP
     valloc();
@@ -652,12 +653,12 @@ elif [ -f /xenix ]; then
   fi
 elif uname -X >/dev/null 2>/dev/null; then
 # SCO shared library check
-  echo "int main() { return 0;}" > conftest.c
+  echo "int main(void) { return 0;}" > conftest.c
   $CC -o conftest conftest.c -lc_s -nointl >/dev/null 2> /dev/null
   [ $? -eq 0 ] && LFLAGS2="-lc_s -nointl"
 else
   SYSTEM=`uname -s 2>/dev/null` || SYSTEM="unknown"
-  echo "int main() { return 0;}" > conftest.c
+  echo "int main(void) { return 0;}" > conftest.c
   case $SYSTEM in
      OSF1|ULTRIX)
         echo Check for -Olimit option