Avoid implicit ints in the configure script, to prevent checks from
failing unconditionally with future compilers that do not support
them by default.

diff --git a/configure b/configure
index 2dd357d4f253946f..cffa76cdb8d68887 100755
--- a/configure
+++ b/configure
@@ -4007,7 +4007,7 @@ $as_echo "#define AC_APPLE_UNIVERSAL_BUILD 1" >>confdefs.h
 
 # System-depending optimization
 printf 'checking for 64-bit availability... '
-if printf 'main() {}' | $CC -xc -m64 -o config.tmp - >config.tmp 2>&1
+if printf 'int main(void) {return 0;}' | $CC -xc -m64 -o config.tmp - >config.tmp 2>&1
 then
   MYCFLAGS="-m64 $MYCFLAGS"
   MYCXXFLAGS="-m64 $MYCXXFLAGS"
@@ -4018,7 +4018,7 @@ fi
 if test "$enable_opt" != "no"
 then
   printf 'checking for CPU optimization availability... '
-  if printf 'main() {}' | $CC -xc -march=native -o config.tmp - >config.tmp 2>&1
+  if printf 'int main(void) {return 0;}' | $CC -xc -march=native -o config.tmp - >config.tmp 2>&1
   then
     MYCFLAGS="-march=native $MYCFLAGS"
     MYCXXFLAGS="-march=native $MYCXXFLAGS"
@@ -4028,7 +4028,7 @@ then
   fi
 fi
 printf 'checking for useless warnings... '
-if printf 'main() {}' | $CC -xc \
+if printf 'int main(void) {return 0;}' | $CC -xc \
   -Wno-unused-but-set-variable -Wno-unused-but-set-parameter -o config.tmp - >config.tmp 2>&1
 then
   MYCFLAGS="$MYCFLAGS -Wno-unused-but-set-variable -Wno-unused-but-set-parameter"
diff --git a/configure.in b/configure.in
index 6ce03e3313a211de..5abb5c33281b9da8 100644
--- a/configure.in
+++ b/configure.in
@@ -204,7 +204,7 @@ AC_C_BIGENDIAN(MYCPPFLAGS="$MYCPPFLAGS -D_MYBIGEND")
 
 # System-depending optimization
 printf 'checking for 64-bit availability... '
-if printf 'main() {}' | $CC -xc -m64 -o config.tmp - >config.tmp 2>&1
+if printf 'int main(void) {return 0;}' | $CC -xc -m64 -o config.tmp - >config.tmp 2>&1
 then
   MYCFLAGS="-m64 $MYCFLAGS"
   MYCXXFLAGS="-m64 $MYCXXFLAGS"
@@ -215,7 +215,7 @@ fi
 if test "$enable_opt" != "no"
 then
   printf 'checking for CPU optimization availability... '
-  if printf 'main() {}' | $CC -xc -march=native -o config.tmp - >config.tmp 2>&1
+  if printf 'int main(void) {return 0;}' | $CC -xc -march=native -o config.tmp - >config.tmp 2>&1
   then
     MYCFLAGS="-march=native $MYCFLAGS"
     MYCXXFLAGS="-march=native $MYCXXFLAGS"
@@ -225,7 +225,7 @@ then
   fi
 fi
 printf 'checking for useless warnings... '
-if printf 'main() {}' | $CC -xc \
+if printf 'int main(void) {return 0;}' | $CC -xc \
   -Wno-unused-but-set-variable -Wno-unused-but-set-parameter -o config.tmp - >config.tmp 2>&1
 then
   MYCFLAGS="$MYCFLAGS -Wno-unused-but-set-variable -Wno-unused-but-set-parameter"