Thank you for your most helpful replies. I did:
1: sudo ldconfig
/sbin/ldconfig.real: /usr/local/lib/libguile-2.0.so.22.7.2-gdb.scm is not an
ELF file - it has the wrong magic bytes at the start.
2: pkg-config --cflags --libs guile-2.0
and now things are working (I have only done some very simple things
to test). I do not have the foggiest idea why it was so easy. In theory,
neither 1 nor 2 should have worked.
1: if gcc really did not know to search in /usr/local/lib I would
have expected all manner of previous successful compiles to fail.
File /etc/ld.so.conf.d/libc.conf (date April 2012) lists /usr/local/lib .
No file in this directory mentions Guile.