https://github.com/kkos/oniguruma/issues/68
https://github.com/kkos/oniguruma/commit/1337755c2758a62057ef95f1cff1b9466f533301

--- /sample/Makefile.am
+++ /sample/Makefile.am
@@ -4,7 +4,7 @@
 LDADD  = $(lib_onig)
 
 AM_LDFLAGS  = -L$(prefix)/lib
-AM_CPPFLAGS = -I../src -I$(includedir)
+AM_CPPFLAGS = -I$(top_srcdir)/src -I$(includedir)
 
 TESTS = encode listcap names posix simple sql syntax user_property bug_fix
 
--- /test/Makefile.am
+++ /test/Makefile.am
@@ -3,7 +3,7 @@
 
 AM_LDFLAGS  = -L$(prefix)/lib
 AM_CFLAGS =
-AM_CPPFLAGS = -I../src -I$(includedir)
+AM_CPPFLAGS = -I$(top_srcdir)/src -I$(includedir)
 
 TESTS = test_utf8 testc testp testcu