Fix typo causing build issues

This looks like a small typo but it was causing 
$ ./autogen.sh 
libtoolize: putting auxiliary files in AC_CONFIG_AUX_DIR, `build'.
libtoolize: copying file `build/ltmain.sh'
libtoolize: putting macros in AC_CONFIG_MACRO_DIR, `build'.
libtoolize: copying file `build/libtool.m4'
libtoolize: copying file `build/ltoptions.m4'
libtoolize: copying file `build/ltsugar.m4'
libtoolize: copying file `build/ltversion.m4'
libtoolize: copying file `build/lt~obsolete.m4'
libtoolize: putting auxiliary files in AC_CONFIG_AUX_DIR, `build'.
libtoolize: copying file `build/config.guess'
libtoolize: copying file `build/config.sub'
libtoolize: copying file `build/install-sh'
configure.ac:18: installing `build/ar-lib'
configure.ac:17: installing `build/missing'
alp2/Makefile.am: installing `build/depcomp'
standalone/Makefile.am:77: whitespace following trailing backslash
Makefile.am:43: whitespace following trailing backslash
standalone/Makefile.am:77: whitespace following trailing backslash
Makefile.am:43: whitespace following trailing backslash

$ sudo dpkg -l | grep -P "  (libtool|automake)"
ii  automake                   1:1.11.3-1ubuntu2            Tool for generating GNU Standards-compliant Makefiles
ii  libtool                         2.4.2-1ubuntu1                Generic library support script
$