configure: use cflags,libs,ldflags of openssl

configure.ac

@@ -146,7 +146,9 @@ AS_IF([test "x$with_openssl" != "xno"],
       [PKG_CHECK_MODULES([OpenSSL], [openssl >= 1.1], [have_openssl=yes], [have_openssl=no])],
       [have_openssl=no])
 AS_IF([test "x$have_openssl" = "xyes"], [
-    LDFLAGS="${LDFLAGS} -lssl"
+    AC_SUBST(OpenSSL_CFLAGS, "")
+    AC_SUBST(OpenSSL_LIBS, "")
+    AC_SUBST(OpenSSL_LDFLAGS, "-lssl")
     AC_MSG_NOTICE([Using OpenSSL])
 ], [
     AC_MSG_NOTICE([Not using OpenSSL])

src/Makefile.am

 lib_LTLIBRARIES = libopendht.la
 
-libopendht_la_CPPFLAGS = @CPPFLAGS@ -I$(top_srcdir)/include/opendht @Argon2_CFLAGS@ @JsonCpp_CFLAGS@ @MsgPack_CFLAGS@
-libopendht_la_LIBADD   = @Argon2_LIBS@ @JsonCpp_LIBS@ @GnuTLS_LIBS@ @Nettle_LIBS@
-libopendht_la_LDFLAGS  = @LDFLAGS@ @Argon2_LDFLAGS@ -version-number @OPENDHT_MAJOR_VERSION@:@OPENDHT_MINOR_VERSION@:@OPENDHT_PATCH_VERSION@
+libopendht_la_CPPFLAGS = @CPPFLAGS@ -I$(top_srcdir)/include/opendht @Argon2_CFLAGS@ @JsonCpp_CFLAGS@ @MsgPack_CFLAGS@ @OpenSSL_CFLAGS@
+libopendht_la_LIBADD   = @Argon2_LIBS@ @JsonCpp_LIBS@ @GnuTLS_LIBS@ @Nettle_LIBS@ @OpenSSL_LIBS@
+libopendht_la_LDFLAGS  = @LDFLAGS@ @Argon2_LDFLAGS@ @OpenSSL_LDFLAGS@ -version-number @OPENDHT_MAJOR_VERSION@:@OPENDHT_MINOR_VERSION@:@OPENDHT_PATCH_VERSION@
 libopendht_la_SOURCES  = \
         dht.cpp \
         storage.h \