Update Built-in Option c_std for C17. Closes #4842.

docs/markdown/Builtin-options.md

@@ -128,7 +128,7 @@ compiler being used:
 | ------       | ------------- | ---------------                          | ----------- |
 | c_args       |               | free-form comma-separated list           | C compile arguments to use |
 | c_link_args  |               | free-form comma-separated list           | C link arguments to use |
-| c_std        | none          | none, c89, c99, c11, gnu89, gnu99, gnu11 | C language standard to use |
+| c_std        | none          | none, c89, c99, c11, c17, c18, gnu89, gnu99, gnu11, gnu17, gnu18 | C language standard to use |
 | c_winlibs    | see below     | free-form comma-separated list           | Standard Windows libs to link against |
 | cpp_args     |               | free-form comma-separated list           | C++ compile arguments to use |
 | cpp_link_args|               | free-form comma-separated list           | C++ link arguments to use |

docs/markdown/Configuring-a-build-directory.md

@@ -60,7 +60,7 @@ sample output for a simple project.
       ------        ------------- ---------------                                                                                               -----------
       c_args        []                                                                                                                          Extra arguments passed to the C compiler
       c_link_args   []                                                                                                                          Extra arguments passed to the C linker
-      c_std         c99           [none, c89, c99, c11, gnu89, gnu99, gnu11]                                                                    C language standard to use
+      c_std         c99           [none, c89, c99, c11, c17, c18, gnu89, gnu99, gnu11, gnu17, gnu18]                                                                    C language standard to use
       cpp_args      []                                                                                                                          Extra arguments passed to the C++ compiler
       cpp_debugstl  false         [true, false]                                                                                                 STL debug mode
       cpp_link_args []                                                                                                                          Extra arguments passed to the C++ linker

docs/markdown/snippets/add_c17_and_c18_standards.md

@@ -0,0 +1,9 @@
+---
+short-description: Add c17 and c18 values for c_std option
+...
+
+## Added c17 and c18 as c_std values for recent GCC and Clang Versions
+
+For gcc version 8.0 and later, the values c17, c18, gnu17, and gnu18 were added to the accepted values for built-in compiler option c_std.
+
+For Clang version 10.0 and later on Apple OSX (Darwin), and for version 7.0 and later on other platforms, the values c17 and gnu17 were added as c_std values.

mesonbuild/compilers/c.py

@@ -1220,9 +1220,17 @@ class ClangCCompiler(ClangCompiler, CCompiler):
 
     def get_options(self):
         opts = CCompiler.get_options(self)
+        c_stds = ['c89', 'c99', 'c11']
+        g_stds = ['gnu89', 'gnu99', 'gnu11']
+        if self.compiler_type is CompilerType.CLANG_OSX:
+            v = '>=10.0.0'
+        else:
+            v = '>=7.0.0'
+        if version_compare(self.version, v):
+            c_stds += ['c17']
+            g_stds += ['gnu17']
         opts.update({'c_std': coredata.UserComboOption('c_std', 'C language standard to use',
-                                                       ['none', 'c89', 'c99', 'c11',
-                                                        'gnu89', 'gnu99', 'gnu11'],
+                                                       ['none'] + c_stds + g_stds,
                                                        'none')})
         return opts
 
@@ -1284,9 +1292,14 @@ class GnuCCompiler(GnuCompiler, CCompiler):
 
     def get_options(self):
         opts = CCompiler.get_options(self)
+        c_stds = ['c89', 'c99', 'c11']
+        g_stds = ['gnu89', 'gnu99', 'gnu11']
+        v = '>=8.0.0'
+        if version_compare(self.version, v):
+            c_stds += ['c17', 'c18']
+            g_stds += ['gnu17', 'gnu18']
         opts.update({'c_std': coredata.UserComboOption('c_std', 'C language standard to use',
-                                                       ['none', 'c89', 'c99', 'c11',
-                                                        'gnu89', 'gnu99', 'gnu11'],
+                                                       ['none'] + c_stds + g_stds,
                                                        'none')})
         if self.compiler_type.is_windows_compiler:
             opts.update({