I'm building libsoup via JHBuild. libsoup builds
successfully. If upstream then updates configure.in, then I run 'make', which
attempts a reconfigure, but fails.

This is because JHBuild sets ACLOCAL_FLAGS to include the sandbox (e.g. -I
/opt/gnome/share/aclocal). One may argue it is silly for JHBuild to require
every module to support ACLOCAL_FLAGS, but there isn't a way around it until
automake-1.12 is out - which includes support for ACLOCAL_PATH. Further details
in the JHBuild bug 634617.

The build failure is:
make  V=1
CDPATH="${ZSH_VERSION+.}:" && cd . && /bin/sh /home/Oxyde/gnome/libsoup/missing --run aclocal-1.11 -I m4
configure.ac:75: warning: macro `AM_PATH_GLIB_2_0' not found in library
configure.ac:6: warning: AC_INIT: not a literal: http://bugzilla.gnome.org/enter_bug.cgi?product=libsoup
 cd . && /bin/sh /home/Oxyde/gnome/libsoup/missing --run automake-1.11 --foreign
configure.ac:6: warning: AC_INIT: not a literal: http://bugzilla.gnome.org/enter_bug.cgi?product=libsoup
CDPATH="${ZSH_VERSION+.}:" && cd . && /bin/sh /home/Oxyde/gnome/libsoup/missing --run autoconf
configure.ac:6: warning: AC_INIT: not a literal: http://bugzilla.gnome.org/enter_bug.cgi?product=libsoup
configure.ac:75: error: possibly undefined macro: AM_PATH_GLIB_2_0
      If this token and others are legitimate, please use m4_pattern_allow.
      See the Autoconf documentation.
make: *** [configure] Error 1