[CMake] Fix -Wstrict-prototypes

Fixes warnings (or errors, if someone injects -Werror in their build system,
which happens in fact with some folks vendoring LLVM too) with Clang 16:
```
+/var/tmp/portage.notmp/portage/sys-devel/llvm-15.0.4/work/llvm_build-abi_x86_64.amd64/CMakeFiles/CMakeTmp/src.c:3:9: warning: a function declaration without a prototype
is deprecated in all versions of C [-Wstrict-prototypes]
-/var/tmp/portage.notmp/portage/sys-devel/llvm-14.0.4/work/llvm_build-abi_x86_64.amd64/CMakeFiles/CMakeTmp/src.c:3:9: error: a function declaration without a prototype is
deprecated in all versions of C [-Werror,-Wstrict-prototypes]
 int main() {return 0;}
         ^
          void
```

Differential Revision: https://reviews.llvm.org/D137503

1 files changed, 12 insertions, 12 deletions

diff --git a/polly/lib/External/CMakeLists.txt b/polly/lib/External/CMakeLists.txt
index 2f912e7d..c0a5b32 100644
--- a/polly/lib/External/CMakeLists.txt
+++ b/polly/lib/External/CMakeLists.txt
@@ -64,7 +64,7 @@ if (POLLY_BUNDLED_ISL)
     check_c_source_compiles("
     ${_includes}
     ${_type} typeVar;
-    int main() {
+    int main(void) {
     return 0;
     }
     " ${_variable})
@@ -73,7 +73,7 @@ if (POLLY_BUNDLED_ISL)
 
   check_c_source_compiles("
   int func(void) __attribute__((__warn_unused_result__));
-  int main() { return 0; }
+  int main(void) { return 0; }
   " HAS_ATTRIBUTE_WARN_UNUSED_RESULT)
   set(GCC_WARN_UNUSED_RESULT)
   if (HAS_ATTRIBUTE_WARN_UNUSED_RESULT)
@@ -82,22 +82,22 @@ if (POLLY_BUNDLED_ISL)
 
   check_c_source_compiles("
   __attribute__ ((unused)) static void foo(void);
-  int main() { return 0; }
+  int main(void) { return 0; }
   " HAVE___ATTRIBUTE__)
 
 
   check_c_source_compiles_numeric("
   #include <strings.h>
-  int main() { (void)ffs(0); return 0; }
+  int main(void) { (void)ffs(0); return 0; }
   " HAVE_DECL_FFS)
 
   check_c_source_compiles_numeric("
-  int main() { (void)__builtin_ffs(0); return 0; }
+  int main(void) { (void)__builtin_ffs(0); return 0; }
   " HAVE_DECL___BUILTIN_FFS)
 
   check_c_source_compiles_numeric("
   #include <intrin.h>
-  int main() { (void)_BitScanForward(NULL, 0); return 0; }
+  int main(void) { (void)_BitScanForward(NULL, 0); return 0; }
   " HAVE_DECL__BITSCANFORWARD)
 
   if (NOT HAVE_DECL_FFS AND
@@ -109,12 +109,12 @@ if (POLLY_BUNDLED_ISL)
 
   check_c_source_compiles_numeric("
   #include <strings.h>
-  int main() { (void)strcasecmp(\"\", \"\"); return 0; }
+  int main(void) { (void)strcasecmp(\"\", \"\"); return 0; }
   " HAVE_DECL_STRCASECMP)
 
   check_c_source_compiles_numeric("
   #include <string.h>
-  int main() { (void)_stricmp(\"\", \"\"); return 0; }
+  int main(void) { (void)_stricmp(\"\", \"\"); return 0; }
   " HAVE_DECL__STRICMP)
 
   if (NOT HAVE_DECL_STRCASECMP AND NOT HAVE_DECL__STRICMP)
@@ -124,12 +124,12 @@ if (POLLY_BUNDLED_ISL)
 
   check_c_source_compiles_numeric("
   #include <strings.h>
-  int main() { (void)strncasecmp(\"\", \"\", 0); return 0; }
+  int main(void) { (void)strncasecmp(\"\", \"\", 0); return 0; }
   " HAVE_DECL_STRNCASECMP)
 
   check_c_source_compiles_numeric("
   #include <string.h>
-  int main() { (void)_strnicmp(\"\", \"\", 0); return 0; }
+  int main(void) { (void)_strnicmp(\"\", \"\", 0); return 0; }
   " HAVE_DECL__STRNICMP)
 
   if (NOT HAVE_DECL_STRNCASECMP AND NOT HAVE_DECL__STRNICMP)
@@ -139,12 +139,12 @@ if (POLLY_BUNDLED_ISL)
 
   check_c_source_compiles_numeric("
   #include <stdio.h>
-  int main() { snprintf((void*)0, 0, \" \"); return 0; }
+  int main(void) { snprintf((void*)0, 0, \" \"); return 0; }
   " HAVE_DECL_SNPRINTF)
 
   check_c_source_compiles_numeric("
   #include <stdio.h>
-  int main() { _snprintf((void*)0, 0, \" \"); return 0; }
+  int main(void) { _snprintf((void*)0, 0, \" \"); return 0; }
   " HAVE_DECL__SNPRINTF)
 
   if (NOT HAVE_DECL_SNPRINTF AND NOT HAVE_DECL__SNPRINTF)