Makefile highlight: fix variable names in "else" conditionals

Summary:
BUG: 417379
FIXED-IN: 5.68.0

Allow highlighting variables in "else" conditionals, for example: else ifdef foo or else ifeq (bar, foo).

Previously, these (bar and foo) were highlighted as "Error".

Reviewers: Framework: Syntax Highlighting, dhaumann, cullmann

Reviewed By: Framework: Syntax Highlighting, cullmann

Subscribers: kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D27343