Use pkg-config-provided LDFLAGS for libsqlite3 and libcurl.

Previously, pkg-config was already queried for libsqlite3's and
libcurl's link flags.  However they were not used, but hardcoded
instead.  This commit replaces the hardcoded LDFLAGS by the ones
provided by pkg-config in a similar pattern as already used for
libsodium.

src/libstore/local.mk

@@ -8,7 +8,7 @@ libstore_SOURCES := $(wildcard $(d)/*.cc)
 
 libstore_LIBS = libutil libformat
 
-libstore_LDFLAGS = -lsqlite3 -lbz2 -lcurl
+libstore_LDFLAGS = $(SQLITE3_LIBS) -lbz2 $(LIBCURL_LIBS)
 
 ifeq ($(OS), SunOS)
 	libstore_LDFLAGS += -lsocket