Fix compatibility for GNU Make (<4.3)

Due to a compatibility-breaking change in GNU Make 4.3, the definition of EXIM4VERSIONLOCALSCAN in debian/rules from commit 8a8bec0a results in the error debian/rules:26: *** unterminated call to function 'shell': missing ')'. Stop. in GNU Make versions below 4.3 due to the unescaped #, resulting in the remainder of the line being commented out.

This commit restores compatibility with versions <4.3 by searching ^.define in place of #define, and accounts for possible changes in whitespace.