Cleanup the way we detect json-c library to use only pkg-config

lib/dns/tests/Makefile.in

@@ -16,7 +16,7 @@ VERSION=@BIND9_VERSION@
 @BIND9_MAKE_INCLUDES@
 
 CINCLUDES =	-I. -Iinclude ${DNS_INCLUDES} ${ISC_INCLUDES} \
-		@OPENSSL_INCLUDES@ @CMOCKA_CFLAGS@
+		@OPENSSL_INCLUDES@ @CMOCKA_CFLAGS@ @JSON_C_CFLAGS@
 CDEFINES =	-DTESTS="\"${top_builddir}/lib/dns/tests/\""
 
 ISCLIBS =	../../isc/libisc.@A@ @OPENSSL_LIBS@
@@ -24,7 +24,7 @@ ISCDEPLIBS =	../../isc/libisc.@A@
 DNSLIBS =	../libdns.@A@ @DNS_CRYPTO_LIBS@
 DNSDEPLIBS =	../libdns.@A@
 
-LIBS =		@LIBS@ @CMOCKA_LIBS@
+LIBS =		@LIBS@ @CMOCKA_LIBS@ @JSON_C_LIBS@
 
 OBJS =		dnstest.@O@
 SRCS =		acl_test.c \