Add a new compiler method: get_return_value()

This method accepts a single function that takes no arguments and
returns a single value which can be a value that can be cast to
a 64-bit signed integer, or a string, and returns that value.

Mostly useful for running foolib_version() functions that return the
currently-available version of libraries.

mesonbuild/compilers/c.py

@@ -482,6 +482,34 @@ class CCompiler(Compiler):
         # minus the extra newline at the end
         return p.stdo.split(delim + '\n')[-1][:-1]
 
+    def get_return_value(self, fname, rtype, prefix, env, extra_args, dependencies):
+        if rtype == 'string':
+            fmt = '%s'
+            cast = '(char*)'
+        elif rtype == 'int':
+            fmt = '%lli'
+            cast = '(long long int)'
+        else:
+            raise AssertionError('BUG: Unknown return type {!r}'.format(rtype))
+        fargs = {'prefix': prefix, 'f': fname, 'cast': cast, 'fmt': fmt}
+        code = '''{prefix}
+        #include <stdio.h>
+        int main(int argc, char *argv[]) {{
+            printf ("{fmt}", {cast} {f}());
+        }}'''.format(**fargs)
+        res = self.run(code, env, extra_args, dependencies)
+        if not res.compiled:
+            m = 'Could not get return value of {}()'
+            raise EnvironmentException(m.format(fname))
+        if rtype == 'string':
+            return res.stdout
+        elif rtype == 'int':
+            try:
+                return int(res.stdout.strip())
+            except:
+                m = 'Return value of {}() is not an int'
+                raise EnvironmentException(m.format(fname))
+
     @staticmethod
     def _no_prototype_templ():
         """

mesonbuild/interpreter.py

@@ -882,7 +882,7 @@ class CompilerHolder(InterpreterObject):
         extra_args = self.determine_args(kwargs)
         deps = self.determine_dependencies(kwargs)
         value = self.compiler.get_define(element, prefix, self.environment, extra_args, deps)
-        mlog.log('Checking for value of define "%s": %s' % (element, value))
+        mlog.log('Fetching value of define "%s": %s' % (element, value))
         return value
 
     def compiles_method(self, args, kwargs):