Don't clobber LDFLAGS user variable

Fix a misunderstanding that's been present since the inception.  The
LDFLAGS variable is entirely at the builder's disposal.  Flags needed
for system-internal settings must go in the AM_* equivalents.

tests/Makefile.am

@@ -2,7 +2,7 @@ AM_CPPFLAGS =					\
 -I$(top_srcdir)					\
 $(OPM_BOOST_CPPFLAGS)
 
-LDFLAGS = $(OPM_BOOST_LDFLAGS)
+AM_LDFLAGS = $(OPM_BOOST_LDFLAGS)
 
 
 LDADD =	$(top_builddir)/lib/libopmcore.la

tutorials/Makefile.am

@@ -2,7 +2,7 @@ AM_CPPFLAGS =					\
 -I$(top_srcdir)					\
 $(OPM_BOOST_CPPFLAGS)
 
-LDFLAGS = $(OPM_BOOST_LDFLAGS)
+AM_LDFLAGS = $(OPM_BOOST_LDFLAGS)
 
 LDADD =	$(top_builddir)/lib/libopmcore.la