By setting 'buildroot' in .jhbuildrc so that jhbuild builds stuffs from outside git source trees, pango fails to build, with the following messages:

---8<---
mv -f .deps/querymodules.Tpo .deps/querymodules.Po
/bin/sh ../libtool  --tag=CC   --mode=link gcc  -g -O2 -I/home/gnome2/include -Wall  -L/home/gnome2/lib64  -o pango-querymodules querymodules.o libpangox-1.0.la libpangoxft-1.0.la libpangoft2-1.0.la  libpango-1.0.la -pthread -L/home/gnome2/lib64 -lgobject-2.0 -lgthread-2.0 -lgmodule-2.0 -lrt -lglib-2.0   
libtool: link: gcc -g -O2 -I/home/gnome2/include -Wall -o .libs/pango-querymodules querymodules.o -pthread  -L/home/gnome2/lib64 ./.libs/libpangox-1.0.so ./.libs/libpangoxft-1.0.so ./.libs/libpangoft2-1.0.so ./.libs/libpango-1.0.so /home/gnome2/lib64/libgobject-2.0.so /home/gnome2/lib64/libgthread-2.0.so /home/gnome2/lib64/libgmodule-2.0.so -lrt /home/gnome2/lib64/libglib-2.0.so -pthread -Wl,-rpath -Wl,/home/gnome2/lib64
make[4]: Leaving directory `/home/thep/build/gnome_git/pango/pango'
make[3]: Leaving directory `/home/thep/build/gnome_git/pango/pango'
make[4]: *** No rule to make target `/home/thep/vcs/gnome_git/pango/pango/pango-enum-types.c', needed by `Pango-1.0.gir'.  Stop.
make[3]: *** [all-recursive] Error 1
make[2]: *** [all] Error 2
make[2]: Leaving directory `/home/thep/build/gnome_git/pango/pango'
make[1]: Leaving directory `/home/thep/build/gnome_git/pango'
make[1]: *** [all-recursive] Error 1
make: *** [all] Error 2
---8<---

It's this line that caused the problem:

> Pango_1_0_gir_FILES = $(addprefix $(srcdir)/,$(pango_introspection_files))

$(pango_introspection_files) includes all $(libpango_1_0_la_SOURCES), including pango-enum-types.c and module-defs-lang.c, which are generated in $(builddir). So, adding prefix $(srcdir) here breaks it.

Note that this kind of problem won't be caught by 'make distcheck', as the generated files are included in the distributed tarball. But this is not the case for building from VCS, such as with jhbuild.