Everywhere: Resolve conflicts with LibC and libc++

Since https://reviews.llvm.org/D131441, libc++ must be included before
LibC. As clang includes libc++ as one of the system includes, LibC
must be included after those, and the only correct way to do that is
to install LibC's headers into the sysroot.

Targets that don't link with LibC yet require its headers for one
reason or another must add install_libc_headers as a dependency to
ensure that the correct headers have been (re)installed into the
sysroot.

LibC/stddef.h has been dropped since the built-in stddef.h receives
a higher include priority.

In addition, string.h and wchar.h must
define __CORRECT_ISO_CPP_STRING_H_PROTO and
_LIBCPP_WCHAR_H_HAS_CONST_OVERLOADS respectively in order to tell
libc++ to not try to define methods implemented by LibC.

Userland/Libraries/LibC/CMakeLists.txt

@@ -76,6 +76,25 @@ set(LIBC_SOURCES
     wstdio.cpp
 )
 
+file(GLOB_RECURSE LIBC_HEADERS RELATIVE "${CMAKE_CURRENT_SOURCE_DIR}" CONFIGURE_DEPENDS "*.h")
+list(APPEND LIBC_HEADERS "../LibELF/ELFABI.h" "../LibRegex/RegexDefs.h")
+
+add_custom_target(install_libc_headers)
+
+# Copy LibC's headers into the sysroot to satisfy libc++'s include priority requirements.
+foreach(RELATIVE_HEADER_PATH IN LISTS LIBC_HEADERS)
+    get_filename_component(directory ${RELATIVE_HEADER_PATH} DIRECTORY)
+    string(REPLACE "../" "" subdirectory "${directory}")
+    file(MAKE_DIRECTORY "${CMAKE_STAGING_PREFIX}/${CMAKE_INSTALL_INCLUDEDIR}/${subdirectory}")
+    add_custom_command(
+        TARGET install_libc_headers
+        PRE_BUILD
+        COMMAND "${CMAKE_COMMAND}" -E copy_if_different "${RELATIVE_HEADER_PATH}" "${CMAKE_STAGING_PREFIX}/${CMAKE_INSTALL_INCLUDEDIR}/${subdirectory}"
+        WORKING_DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}"
+        VERBATIM
+    )
+endforeach()
+
 file(GLOB ELF_SOURCES CONFIGURE_DEPENDS "../LibELF/*.cpp")
 
 if ("${SERENITY_ARCH}" STREQUAL "aarch64")
@@ -98,12 +117,14 @@ set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-unknown-warning-option")
 # Since all native serenity applications use dynamic libraries, prevent coverage on libc.a as well
 
 add_library(crt0 STATIC crt0.cpp)
+add_dependencies(crt0 install_libc_headers)
 target_link_libraries(crt0 PRIVATE NoCoverage)
 add_custom_command(
     TARGET crt0
     COMMAND "${CMAKE_COMMAND}" -E copy $<TARGET_OBJECTS:crt0> ${CMAKE_INSTALL_PREFIX}/usr/lib/crt0.o
 )
 add_library(crt0_shared STATIC crt0_shared.cpp)
+add_dependencies(crt0_shared install_libc_headers)
 target_link_libraries(crt0_shared PRIVATE NoCoverage)
 add_custom_command(
     TARGET crt0_shared
@@ -126,6 +147,7 @@ add_custom_command(
 
 set_source_files_properties (ssp_nonshared.cpp PROPERTIES COMPILE_FLAGS "-fno-stack-protector")
 add_library(ssp_nonshared STATIC ssp_nonshared.cpp)
+add_dependencies(ssp_nonshared install_libc_headers)
 target_link_libraries(ssp_nonshared PRIVATE NoCoverage)
 install(FILES ${CMAKE_CURRENT_BINARY_DIR}/libssp_nonshared.a DESTINATION ${CMAKE_INSTALL_PREFIX}/usr/lib/)
 
@@ -164,7 +186,7 @@ set_property(
 
 set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -nolibc")
 serenity_libc(LibC c)
-add_dependencies(LibC crti crt0 crt0_shared crtn)
+add_dependencies(LibC crti crt0 crt0_shared crtn install_libc_headers)
 target_link_libraries(LibC PRIVATE LibSystem LibTimeZone)
 
 # We mark LibCStatic as a dependency of LibC because this triggers the build of the LibCStatic target

Userland/Libraries/LibC/stddef.h

@@ -1,20 +0,0 @@
-/*
- * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
- *
- * SPDX-License-Identifier: BSD-2-Clause
- */
-
-#pragma once
-
-#include <sys/cdefs.h>
-
-#define offsetof(type, member) __builtin_offsetof(type, member)
-
-#ifdef __cplusplus
-#    define NULL nullptr
-#else
-#    define NULL ((void*)0)
-#endif
-
-typedef __PTRDIFF_TYPE__ ptrdiff_t;
-typedef __SIZE_TYPE__ size_t;

Userland/Libraries/LibC/string.h

@@ -12,6 +12,10 @@
 
 __BEGIN_DECLS
 
+#ifdef __cplusplus
+#    define __CORRECT_ISO_CPP_STRING_H_PROTO
+#endif
+
 // A few C Standard Libraries include this header in <string.h>, and hence expect
 // `strcasecmp` etcetera to be available as part of a <string.h> include, so let's
 // do the same here to maintain compatibility

Userland/Libraries/LibC/wchar.h

@@ -18,6 +18,10 @@ __BEGIN_DECLS
 #    define WEOF (0xffffffffu)
 #endif
 
+#ifdef __cplusplus
+#    define _LIBCPP_WCHAR_H_HAS_CONST_OVERLOADS
+#endif
+
 typedef __WINT_TYPE__ wint_t;
 typedef unsigned long int wctype_t;