Broken cross-compilation

Cross-compilation of mutt is broken since commit 9bacbaa9 because even with --disable-doc, mutt tries to generate and install documentation files.

Build fails because mutt wants to generate muttrc.man by building (and then running) a makedoc binary with the target cross-compiler (which will obviously fail).

As a result, build fails on:

/home/fabrice/buildroot/output/host/bin/arm-none-linux-gnueabi-gcc -std=gnu99 -DSYSCONFDIR=\"/etc\" -DBINDIR=\"/usr/bin\" -DHAVE_CONFIG_H=1 -I. -I..  -I. -I.. -I/usr/include -I.. -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -I../intl  -Wall -pedantic -Wno-long-long -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64  -Os   -c -o makedoc.o makedoc.c
sed -e 's,@sysconfdir\@,/etc,g' -e 's,@bindir\@,/usr/bin,g' -e 's,@docdir\@,/usr/share/doc/mutt,g' ./mutt.man > mutt.1
sed -e 's,@sysconfdir\@,/etc,g' -e 's,@bindir\@,/usr/bin,g' -e 's,@docdir\@,/usr/share/doc/mutt,g' ./smime_keys.man > smime_keys.1
sed -e 's,@sysconfdir\@,/etc,g' -e 's,@bindir\@,/usr/bin,g' -e 's,@docdir\@,/usr/share/doc/mutt,g' ./pgpewrap.man > pgpewrap.1
sed -e 's,@sysconfdir\@,/etc,g' -e 's,@bindir\@,/usr/bin,g' -e 's,@docdir\@,/usr/share/doc/mutt,g' ./pgpring.man > mutt_pgpring.1
arm-none-linux-gnueabi-gcc: ERROR: unsafe header/library path used in cross-compilation: '-I/usr/include'

I would advise to revert this commit as it does not seem right to build and install doc when the user has rightfully disable it through --disable-doc.