Add string underscorify() function

So we can easily construct the defines for include headers and
struct checks and such.

mesonbuild/interpreter.py

@@ -2037,6 +2037,8 @@ class Interpreter():
             return obj.upper()
         elif method_name == 'to_lower':
             return obj.lower()
+        elif method_name == 'underscorify':
+            return re.sub(r'[^a-zA-Z0-9]', '_', obj)
         elif method_name == 'split':
             if len(posargs) > 1:
                 raise InterpreterException('Split()  must have at most one argument.')

test cases/common/42 string formatting/meson.build

@@ -65,4 +65,17 @@ if long.to_upper().to_lower() != long
   error('Broken to_lower')
 endif
 
+if 'struct stat.st_foo'.underscorify() != 'struct_stat_st_foo'
+  error('Broken underscorify')
+endif
+
+if '#include <foo/bar.h>'.underscorify() != '_include__foo_bar_h_'
+  error('Broken underscorify')
+endif
+
+# case should not change, space should be replaced, numbers are ok too
+if 'Do SomeThing 09'.underscorify() != 'Do_SomeThing_09'
+  error('Broken underscorify')
+endif
+
 assert('3'.to_int() == 3, 'String int conversion does not work.')