configure: extract top_srcdir from Makefile at external of quotes
configure: extract top_srcdir from Makefile at external of quotes

This fixes the following error on pdksh v5.2.14.

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sed: -e expression #1, char 1: unknown command: `"'
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Signed-off-by: Rafaël Carré <29a40ef51885229607cd1e19e467be5d13190cad@videolan.org>

krichter722/vlc,krichter722/vlc,xkfz007/vlc,vlc-mirror/vlc,vlc-mirror/vlc,xkfz007/vlc,krichter722/vlc,shyamalschandra/vlc,krichter722/vlc,xkfz007/vlc,krichter722/vlc,xkfz007/vlc,xkfz007/vlc,krichter722/vlc,vlc-mirror/vlc,shyamalschandra/vlc,shyamalschandra/vlc,krichter722/vlc,xkfz007/vlc,shyamalschandra/vlc,vlc-mirror/vlc,vlc-mirror/vlc,vlc-mirror/vlc,vlc-mirror/vlc,shyamalschandra/vlc,shyamalschandra/vlc,xkfz007/vlc,shyamalschandra/vlc