Modules: Fix checks under -Werror=strict-prototypes

This fixes configuration checks when using -Werror=strict-prototypes or
-Wstrict-prototypes compiler flag, for example, via the CMAKE_C_FLAGS.

Modules/CheckStructHasMember.cmake

@@ -66,7 +66,7 @@ macro (CHECK_STRUCT_HAS_MEMBER _STRUCT _MEMBER _HEADER _RESULT)
 
   set(_CHECK_STRUCT_MEMBER_SOURCE_CODE "
 ${_INCLUDE_FILES}
-int main()
+int main(void)
 {
   (void)sizeof(((${_STRUCT} *)0)->${_MEMBER});
   return 0;

Modules/FindIconv.cmake

@@ -101,7 +101,7 @@ if(NOT DEFINED Iconv_IS_BUILT_IN)
       "
       #include <stddef.h>
       #include <iconv.h>
-      int main() {
+      int main(void) {
         char *a, *b;
         size_t i, j;
         iconv_t ic;

Modules/FindOpenACC.cmake

@@ -72,7 +72,7 @@ be returned with OpenACC_<lang>_FLAGS.
 
 set(OpenACC_C_CXX_TEST_SOURCE
 "
-int main(){
+int main(void){
 #ifdef _OPENACC
   return 0;
 #else
@@ -102,7 +102,7 @@ const char accver_str[] = { 'I', 'N', 'F', 'O', ':', 'O', 'p', 'e', 'n', 'A',
                             ('0' + ((_OPENACC/10)%10)),
                             ('0' + ((_OPENACC/1)%10)),
                             ']', '\\0' };
-int main()
+int main(void)
 {
   puts(accver_str);
   return 0;

Modules/GenerateExportHeader.cmake

@@ -210,8 +210,8 @@ endmacro()
 
 # TODO: Install this macro separately?
 macro(_check_c_compiler_attribute _ATTRIBUTE _RESULT)
-  check_source_compiles(C "${_ATTRIBUTE} int somefunc() { return 0; }
-    int main() { return somefunc();}" ${_RESULT}
+  check_source_compiles(C "${_ATTRIBUTE} int somefunc(void) { return 0; }
+    int main(void) { return somefunc();}" ${_RESULT}
   )
 endmacro()