Makefile.am: Don't define LIBEXECDIR

As README points out, the library should not execute out-of-process
tools.  Thus, it should never need to know LIBEXECDIR.
diff --git a/Makefile.am b/Makefile.am
index aa53b51..d7dcaed 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -6,7 +6,6 @@
 AM_CPPFLAGS = \
 	-include $(top_builddir)/config.h \
 	-DSYSCONFDIR=\""$(sysconfdir)"\" \
-	-DLIBEXECDIR=\""$(libexecdir)"\" \
 	-I${top_srcdir}/src
 
 AM_CFLAGS = ${my_CFLAGS} \