[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

(cherry picked from commit 32a2af4)

Author: thesamesam

compiler-rt/cmake/Modules/CompilerRTDarwinUtils.cmake

@@ -116,7 +116,7 @@ function(darwin_test_archs os valid_archs)
   if(NOT TEST_COMPILE_ONLY)
     message(STATUS "Finding valid architectures for ${os}...")
     set(SIMPLE_C ${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/src.c)
-    file(WRITE ${SIMPLE_C} "#include <stdio.h>\nint main() { printf(__FILE__); return 0; }\n")
+    file(WRITE ${SIMPLE_C} "#include <stdio.h>\nint main(void) { printf(__FILE__); return 0; }\n")
 
     set(os_linker_flags)
     foreach(flag ${DARWIN_${os}_LINK_FLAGS})

compiler-rt/cmake/config-ix.cmake

@@ -209,7 +209,7 @@ set(COMPILER_RT_SUPPORTED_ARCH)
 # runtime libraries supported by our current compilers cross-compiling
 # abilities.
 set(SIMPLE_SOURCE ${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/simple.cc)
-file(WRITE ${SIMPLE_SOURCE} "#include <stdlib.h>\n#include <stdio.h>\nint main() { printf(\"hello, world\"); }\n")
+file(WRITE ${SIMPLE_SOURCE} "#include <stdlib.h>\n#include <stdio.h>\nint main(void) { printf(\"hello, world\"); }\n")
 
 # Detect whether the current target platform is 32-bit or 64-bit, and setup
 # the correct commandline flags needed to attempt to target 32-bit and 64-bit.

compiler-rt/lib/builtins/CMakeLists.txt

@@ -745,7 +745,7 @@ else ()
                            SOURCE "#if !(__ARM_FP & 0x8)
                                    #error No double-precision support!
                                    #endif
-                                   int main() { return 0; }")
+                                   int main(void) { return 0; }")
           if(NOT COMPILER_RT_HAS_${arch}_VFP_DP)
             list(REMOVE_ITEM ${arch}_SOURCES ${arm_Thumb1_VFPv2_DP_SOURCES})
           endif()

libcxx/cmake/config-ix.cmake

@@ -94,7 +94,7 @@ if(CMAKE_CXX_COMPILER_ID MATCHES "Clang")
   set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -Werror=unknown-pragmas")
   check_c_source_compiles("
 #pragma comment(lib, \"c\")
-int main() { return 0; }
+int main(void) { return 0; }
 " C_SUPPORTS_COMMENT_LIB_PRAGMA)
   cmake_pop_check_state()
 endif()

libcxxabi/cmake/config-ix.cmake

@@ -77,7 +77,7 @@ if(CMAKE_CXX_COMPILER_ID MATCHES "Clang")
   set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -Werror=unknown-pragmas")
   check_c_source_compiles("
 #pragma comment(lib, \"c\")
-int main() { return 0; }
+int main(void) { return 0; }
 " C_SUPPORTS_COMMENT_LIB_PRAGMA)
   cmake_pop_check_state()
 endif()

libunwind/cmake/config-ix.cmake

@@ -85,7 +85,7 @@ if(CMAKE_CXX_COMPILER_ID MATCHES "Clang")
   set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -Werror=unknown-pragmas")
   check_c_source_compiles("
 #pragma comment(lib, \"c\")
-int main() { return 0; }
+int main(void) { return 0; }
 " C_SUPPORTS_COMMENT_LIB_PRAGMA)
   cmake_pop_check_state()
 endif()

lldb/tools/debugserver/source/CMakeLists.txt

@@ -95,7 +95,7 @@ check_c_source_compiles(
     #else
     #error Not building for ARM64
     #endif
-    int main() { return 0; }
+    int main(void) { return 0; }
     "
     BUILDING_FOR_ARM64_OSX
 )

llvm/cmake/config-ix.cmake

@@ -71,7 +71,7 @@ if(APPLE)
   CHECK_C_SOURCE_COMPILES("
      static const char *__crashreporter_info__ = 0;
      asm(\".desc ___crashreporter_info__, 0x10\");
-     int main() { return 0; }"
+     int main(void) { return 0; }"
     HAVE_CRASHREPORTER_INFO)
 endif()
 

llvm/cmake/modules/FindFFI.cmake

@@ -45,7 +45,7 @@ if(FFI_LIBRARIES)
     struct ffi_cif;
     typedef struct ffi_cif ffi_cif;
     void ffi_call(ffi_cif *cif, void (*fn)(void), void *rvalue, void **avalue);
-    int main() { ffi_call(0, 0, 0, 0); }"
+    int main(void) { ffi_call(0, 0, 0, 0); }"
     HAVE_FFI_CALL)
   cmake_pop_check_state()
 endif()

llvm/cmake/modules/FindTerminfo.cmake

@@ -20,7 +20,7 @@ if(Terminfo_LIBRARIES)
   list(APPEND CMAKE_REQUIRED_LIBRARIES ${Terminfo_LIBRARIES})
   check_c_source_compiles("
     int setupterm(char *term, int filedes, int *errret);
-    int main() { return setupterm(0, 0, 0); }"
+    int main(void) { return setupterm(0, 0, 0); }"
     Terminfo_LINKABLE)
   cmake_pop_check_state()
 endif()