imv-swid: Use pkg-config to check for libjson-c

The package/library is called libjson-c on recent distributions.
Some like Ubuntu 14.04 provide symlinks with the old name but these
will eventually disappear.  Using pkg-config allows us to easily check
for it (with a fallback) and configure the proper compiler flags.

Fixes #663.

configure.ac

@@ -876,8 +876,10 @@ if test x$tss = xtrousers; then
 fi
 
 if test x$imv_swid = xtrue; then
-	AC_CHECK_LIB([json],[main],[LIBS="$LIBS"],[AC_MSG_ERROR([JSON library libjson not found])],[])
-	AC_CHECK_HEADER([json/json.h],,[AC_MSG_ERROR([JSON header json/json.h not found!])])
+	PKG_CHECK_MODULES(json, [json-c], [],
+		[PKG_CHECK_MODULES(json, [json])])
+	AC_SUBST(json_CFLAGS)
+	AC_SUBST(json_LIBS)
 fi
 
 if test x$dumm = xtrue; then

src/libpts/plugins/imv_swid/Makefile.am

@@ -5,7 +5,7 @@ AM_CPPFLAGS = \
 	-I$(top_srcdir)/src/libpts
 
 AM_CFLAGS = \
-	$(PLUGIN_CFLAGS)
+	$(PLUGIN_CFLAGS) $(json_CFLAGS)
 
 imcv_LTLIBRARIES = imv-swid.la
 
@@ -13,7 +13,7 @@ imv_swid_la_LIBADD = \
 	$(top_builddir)/src/libimcv/libimcv.la \
 	$(top_builddir)/src/libpts/libpts.la \
 	$(top_builddir)/src/libstrongswan/libstrongswan.la \
-	-ljson
+	$(json_LIBS)
 
 imv_swid_la_SOURCES = \
 	imv_swid.c imv_swid_state.h imv_swid_state.c \

src/libpts/plugins/imv_swid/imv_swid_rest.h

@@ -26,7 +26,7 @@
 
 #include <library.h>
 
-#include <json/json.h>
+#include <json.h>
 
 typedef struct imv_swid_rest_t imv_swid_rest_t;
 

src/libpts/plugins/imv_swid/imv_swid_state.h

@@ -28,7 +28,7 @@
 #include <swid/swid_inventory.h>
 #include <library.h>
 
-#include <json/json.h>
+#include <json.h>
 
 typedef struct imv_swid_state_t imv_swid_state_t;
 typedef enum imv_swid_handshake_state_t imv_swid_handshake_state_t;