mutt: always install shipped manual.txt

If docbook and xslt is install the makefile recreate manual.html but
fails to create manuall.txt if no elinks, lynks or w3m is installed.
Instead it installs an empty file.

Problem reported by Svyatoslav Mishyn.

mutt/Pkgfile

@@ -11,6 +11,8 @@ source=(https://bitbucket.org/mutt/mutt/downloads/$name-$version.tar.gz)
 build () {
     cd $name-$version
 
+    sed -ri '/^makedoc-all/s/manual\.(html|txt)//g' doc/Makefile.am
+
     ./configure --prefix=/usr \
                 --with-docdir=/usr/share/mutt \
                 --with-mailpath=/var/spool/mail \