Port the ccc build configuration detection tool to C99 by
specifying the return type of main as int.

Submitted upstream:

  <https://groups.google.com/g/lp_solve/c/WjVf0dxrwfQ/m/rKMwf57tAwAJ>

diff --git a/lp_solve/ccc b/lp_solve/ccc
index 9b79cea7ee71536c..b1f761aab33345d4 100644
--- a/lp_solve/ccc
+++ b/lp_solve/ccc
@@ -6,7 +6,7 @@ c=$CC
 >/tmp/platform.c
 echo '#include <stdlib.h>'>>/tmp/platform.c
 echo '#include <stdio.h>'>>/tmp/platform.c
-echo 'main(){printf("ux%d", (int) (sizeof(void *)*8));}'>>/tmp/platform.c
+echo 'int main(){printf("ux%d", (int) (sizeof(void *)*8));}'>>/tmp/platform.c
 $c /tmp/platform.c -o /tmp/platform
 PLATFORM=`/tmp/platform`
 rm /tmp/platform /tmp/platform.c >/dev/null 2>&1
@@ -20,7 +20,7 @@ math=-lm
 echo '#include <stdio.h>'>>/tmp/isnan.c
 echo '#include <stdlib.h>'>>/tmp/isnan.c
 echo '#include <math.h>'>>/tmp/isnan.c
-echo 'main(){isnan(0.0);}'>>/tmp/isnan.c
+echo 'int main(){isnan(0.0);}'>>/tmp/isnan.c
 $c /tmp/isnan.c -o /tmp/isnan $math >/dev/null 2>&1
 if [ $? = 0 ]
 then NOISNAN=
diff --git a/lpsolve55/ccc b/lpsolve55/ccc
index 0470611bc45a851c..ca350f257add7730 100644
--- a/lpsolve55/ccc
+++ b/lpsolve55/ccc
@@ -6,7 +6,7 @@ c=$CC
 >/tmp/platform.c
 echo '#include <stdlib.h>'>>/tmp/platform.c
 echo '#include <stdio.h>'>>/tmp/platform.c
-echo 'main(){printf("ux%d", (int) (sizeof(void *)*8));}'>>/tmp/platform.c
+echo 'int main(){printf("ux%d", (int) (sizeof(void *)*8));}'>>/tmp/platform.c
 $c /tmp/platform.c -o /tmp/platform
 PLATFORM=`/tmp/platform`
 rm /tmp/platform /tmp/platform.c >/dev/null 2>&1
@@ -18,7 +18,7 @@ mkdir bin bin/$PLATFORM >/dev/null 2>&1
 echo '#include <stdio.h>'>>/tmp/isnan.c
 echo '#include <stdlib.h>'>>/tmp/isnan.c
 echo '#include <math.h>'>>/tmp/isnan.c
-echo 'main(){isnan(0.0);}'>>/tmp/isnan.c
+echo 'int main(){isnan(0.0);}'>>/tmp/isnan.c
 $c /tmp/isnan.c -o /tmp/isnan $math >/dev/null 2>&1
 if [ $? = 0 ]
 then NOISNAN=