Why did this compile fail?

[Bruce Korb]

gcc -DHAVE_CONFIG_H -I. -I../../agen5 -I..  -I.. -I../.. -I../autoopts
-I../../autoopts  -pthread -I/u/ROOT/usr/local/include/guile/3.0
-I/u/ROOT/usr/local/include -g -O2 -Wno-format-contains-nul
-fno-strict-aliasing -MT autogen-ag.o -MD -MP -MF .deps/autogen-ag.Tpo
-c -o autogen-ag.o `test -f 'ag.c' || echo '../../agen5/'`ag.c
In file included from /u/ROOT/usr/local/include/guile/3.0/libguile.h:52:0,
                 from ../../agen5/autogen.h:60,
                 from ag.c:9:
/u/ROOT/usr/local/include/guile/3.0/libguile/extensions.h:26:10: fatal
error: libguile/libpath.h: No such file or directory
 #include "libguile/libpath.h"
          ^~~~~~~~~~~~~~~~~~~~
compilation terminated.


The "-I /u/ROOT/usr/local/include/guile/3.0" was used to find
"libguile/extensions.h", so why would
libguile/libpath.h not be found? Why would it not have been found
after a full build and "make install"?