mu: add soft dependency on json-glib

If it's available it will be used.

lib/Makefile.am

@@ -20,12 +20,21 @@ include $(top_srcdir)/gtest.mk
 # before decending into tests/
 SUBDIRS= parser . tests
 
+if HAVE_JSON_GLIB
+json_srcs=					\
+	mu-msg-json.c				\
+	mu-msg-json.h
+json_flag="-DHAVE_JSON_GLIB"
+endif
+
 AM_CFLAGS=					\
 	$(WARN_CFLAGS)				\
 	$(GMIME_CFLAGS)				\
 	$(GLIB_CFLAGS)				\
 	$(GUILE_CFLAGS)				\
+	$(JSON_GLIB_CFLAGS)			\
 	$(ASAN_CFLAGS)				\
+	$(json_flag)				\
 	-Wno-format-nonliteral			\
 	-Wno-switch-enum			\
 	-Wno-suggest-attribute=format		\
@@ -36,6 +45,7 @@ AM_CXXFLAGS=					\
 	$(WARN_CXXFLAGS)			\
 	$(GMIME_CFLAGS)				\
 	$(GLIB_CFLAGS)				\
+	$(JSON_GLIB_CFLAGS)			\
 	$(XAPIAN_CXXFLAGS)			\
 	$(GUILE_CFLAGS)				\
 	$(ASAN_CXXFLAGS)
@@ -76,6 +86,7 @@ libmu_la_SOURCES=				\
 	mu-msg-file.h				\
 	mu-msg-iter.cc				\
 	mu-msg-iter.h				\
+	$(json_srcs)				\
 	mu-msg-part.c				\
 	mu-msg-part.h				\
 	mu-msg-prio.c				\
@@ -108,6 +119,7 @@ libmu_la_LIBADD=				\
 	$(GMIME_LIBS)				\
 	$(GLIB_LIBS)				\
 	$(GUILE_LIBS)				\
+	$(JSON_GLIB_LIBS)			\
 	${builddir}/parser/libmuxparser.la
 
 libmu_la_LDFLAGS=				\

lib/tests/Makefile.am

@@ -32,6 +32,7 @@ AM_CPPFLAGS=$(XAPIAN_CXXFLAGS)					\
 # use -Wno-unused-parameters, because some callbacks may not
 # really need all the params they get
 AM_CFLAGS=							\
+	$(JSON_GLIB_CFLAGS)					\
 	$(WARN_CFLAGS)						\
 	$(ASAN_CFLAGS)
 AM_CXXFLAGS=							\