tests/37 has header: Also test the fallback include check

Also forcibly undefine __has_include and test that the fallback include
check in cc.has_header() works.

This is important because all the latest compilers support it now
and we might have no test coverage at all by accident. GCC 5, ICC 17,
Clang 3.8, and VS2015 Update 2 already support it.

test cases/common/37 has header/meson.build

@@ -9,16 +9,17 @@ configure_file(input : non_existant_header,
   output : non_existant_header,
   configuration : configuration_data())
 
-foreach comp : [meson.get_compiler('c'), meson.get_compiler('cpp')]
-  if not comp.has_header('stdio.h')
-    error('Stdio missing.')
-  endif
+# Test that the fallback to __has_include also works on all compilers
+args = [[], ['-U__has_include']]
+
+foreach arg : args
+  foreach comp : [meson.get_compiler('c'), meson.get_compiler('cpp')]
+    assert(comp.has_header('stdio.h', args : arg), 'Stdio missing.')
 
     # stdio.h doesn't actually need stdlib.h, but just test that setting the
     # prefix does not result in an error.
-  if not comp.has_header('stdio.h', prefix : '#include <stdlib.h>')
-    error('Stdio missing.')
-  endif
+    assert(comp.has_header('stdio.h', prefix : '#include <stdlib.h>', args : arg),
+           'Stdio missing.')
 
     # XInput.h should not require type definitions from windows.h, but it does
     # require macro definitions. Specifically, it requires an arch setting for
@@ -26,22 +27,23 @@ foreach comp : [meson.get_compiler('c'), meson.get_compiler('cpp')]
     # We only do this check on MSVC because MinGW often defines its own wrappers
     # that pre-include windows.h
     if comp.get_id() == 'msvc'
-    if not comp.has_header('XInput.h', prefix : '#include <windows.h>')
-      error('XInput.h should not be missing on Windows')
-    endif
-    if not comp.has_header('XInput.h', prefix : '#define _X86_')
-      error('XInput.h should not need windows.h')
-    endif
+      assert(comp.has_header('XInput.h', prefix : '#include <windows.h>', args : arg),
+             'XInput.h should not be missing on Windows')
+      assert(comp.has_header('XInput.h', prefix : '#define _X86_', args : arg),
+             'XInput.h should not need windows.h')
     endif
 
     # Test that the following GCC bug doesn't happen:
     # https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80005
     # https://github.com/mesonbuild/meson/issues/1458
     if host_system == 'linux'
-    assert(comp.has_header('linux/if.h'), 'Could not find <linux/if.h>')
+      assert(comp.has_header('linux/if.h', args : arg),
+             'Could not find <linux/if.h>')
     endif
 
     # This header exists in the source and the builddir, but we still must not
     # find it since we are looking in the system directories.
-  assert(not comp.has_header(non_existant_header), 'Found non-existant header.')
+    assert(not comp.has_header(non_existant_header, args : arg),
+           'Found non-existant header.')
+  endforeach
 endforeach