Concatenate string literals in get_define

If get_define returns a list of string_literals concatenate them into
one string.

mesonbuild/compilers/c.py

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-import subprocess, os.path
+import subprocess, os.path, ast
 
 from .. import mlog
 from .. import coredata
@@ -567,8 +567,9 @@ class CCompiler(Compiler):
             if p.returncode != 0:
                 raise EnvironmentException('Could not get define {!r}'.format(dname))
         # Get the preprocessed value after the delimiter,
-        # minus the extra newline at the end
-        return p.stdo.split(delim + '\n')[-1][:-1]
+        # minus the extra newline at the end and
+        # merge string literals.
+        return ast.literal_eval(p.stdo.split(delim + '\n')[-1][:-1])
 
     def get_return_value(self, fname, rtype, prefix, env, extra_args, dependencies):
         if rtype == 'string':