CMake compilation: do not use compiler extensions

For now, compiler extensions are no longer activated on CMake sources.
However these extensions are still used for various third parties.

This MR is a partial answer to the issue #20454.

7 files changed, 57 insertions, 0 deletions

diff --git a/Source/CMakeLists.txt b/Source/CMakeLists.txt
index 467abe9..564e647 100644
--- a/Source/CMakeLists.txt
+++ b/Source/CMakeLists.txt
@@ -1,6 +1,11 @@
 # Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
 # file Copyright.txt or https://cmake.org/licensing for details.
 
+# To ensure maximum portability across various compilers and platforms
+# deactivate any compiler extensions
+set(CMAKE_C_EXTENSIONS FALSE)
+set(CMAKE_CXX_EXTENSIONS FALSE)
+
 include(CheckIncludeFile)
 # Check if we can build support for ELF parsing.
 if(CMAKE_CXX_PLATFORM_ID MATCHES "OpenBSD")
diff --git a/Source/CPack/WiX/cmWIXFilesSourceWriter.cxx b/Source/CPack/WiX/cmWIXFilesSourceWriter.cxx
index c0d879a..b4085d5 100644
--- a/Source/CPack/WiX/cmWIXFilesSourceWriter.cxx
+++ b/Source/CPack/WiX/cmWIXFilesSourceWriter.cxx
@@ -1,5 +1,10 @@
 /* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
    file Copyright.txt or https://cmake.org/licensing for details.  */
+#if defined(__CYGWIN__)
+// For S_IWRITE symbol
+#  define _DEFAULT_SOURCE
+#endif
+
 #include "cmWIXFilesSourceWriter.h"
 
 #include "cm_sys_stat.h"
diff --git a/Source/cmLoadCommandCommand.cxx b/Source/cmLoadCommandCommand.cxx
index 92258e2..5790e16 100644
--- a/Source/cmLoadCommandCommand.cxx
+++ b/Source/cmLoadCommandCommand.cxx
@@ -1,5 +1,15 @@
 /* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
    file Copyright.txt or https://cmake.org/licensing for details.  */
+
+#if !defined(_WIN32) && !defined(__sun)
+// POSIX APIs are needed
+#  define _POSIX_C_SOURCE 200809L
+#endif
+#if defined(__OpenBSD__) || defined(__FreeBSD__) || defined(__NetBSD__)
+// For isascii
+#  define _XOPEN_SOURCE 700
+#endif
+
 #include "cmLoadCommandCommand.h"
 
 #include <csignal>
diff --git a/Source/cmStandardLexer.h b/Source/cmStandardLexer.h
index 13f7622..55d23c1 100644
--- a/Source/cmStandardLexer.h
+++ b/Source/cmStandardLexer.h
@@ -3,6 +3,19 @@
 #ifndef cmStandardLexer_h
 #define cmStandardLexer_h
 
+#if !defined(_WIN32) && !defined(__sun)
+/* POSIX APIs are needed */
+#  define _POSIX_C_SOURCE 200809L
+#endif
+#if defined(__sun) && defined(__GNUC__) && !defined(__cplusplus)
+/* C sources: for fileno and strdup */
+#  define _XOPEN_SOURCE 600
+#endif
+#if defined(__OpenBSD__) || defined(__FreeBSD__) || defined(__NetBSD__)
+/* For isascii */
+#  define _XOPEN_SOURCE 700
+#endif
+
 #include "cmsys/Configure.h" // IWYU pragma: keep
 
 /* Disable some warnings.  */
diff --git a/Source/cmSystemTools.cxx b/Source/cmSystemTools.cxx
index d8cd705..81f76ca 100644
--- a/Source/cmSystemTools.cxx
+++ b/Source/cmSystemTools.cxx
@@ -1,5 +1,15 @@
 /* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
    file Copyright.txt or https://cmake.org/licensing for details.  */
+
+#if !defined(_WIN32) && !defined(__sun)
+// POSIX APIs are needed
+#  define _POSIX_C_SOURCE 200809L
+#endif
+#if defined(__OpenBSD__) || defined(__FreeBSD__) || defined(__NetBSD__)
+// For isascii
+#  define _XOPEN_SOURCE 700
+#endif
+
 #include "cmSystemTools.h"
 
 #include <cmext/algorithm>
diff --git a/Source/cmTimestamp.cxx b/Source/cmTimestamp.cxx
index 390fd16..13f73dc 100644
--- a/Source/cmTimestamp.cxx
+++ b/Source/cmTimestamp.cxx
@@ -1,5 +1,15 @@
 /* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
    file Copyright.txt or https://cmake.org/licensing for details.  */
+
+#if !defined(_WIN32) && !defined(__sun)
+// POSIX APIs are needed
+#  define _POSIX_C_SOURCE 200809L
+#endif
+#if defined(__OpenBSD__) || defined(__FreeBSD__) || defined(__NetBSD__)
+// For isascii
+#  define _XOPEN_SOURCE 700
+#endif
+
 #include "cmTimestamp.h"
 
 #include <cstdlib>
diff --git a/Utilities/std/CMakeLists.txt b/Utilities/std/CMakeLists.txt
index 63c0a60..a72abb7 100644
--- a/Utilities/std/CMakeLists.txt
+++ b/Utilities/std/CMakeLists.txt
@@ -1,4 +1,8 @@
 
+# To ensure maximum portability across various compilers and platforms
+# deactivate any compiler extensions
+set(CMAKE_CXX_EXTENSIONS FALSE)
+
 # source files for CMake std library
 set(SRCS cm/bits/string_view.cxx
          cm/memory