daemon: Fix linking gcrypt when --as-needed linker arg is used

This is a followup to 8a7bd211d2.

As it is mentioned in autoconf manual that library names should be
specified in LIBS, not LDFLAGS. See:

https://www.gnu.org/savannah-checkouts/gnu/autoconf/manual/autoconf-2.72/html_node/Preset-Output-Variables.html#index-LDFLAGS-2

This change also brings back the save_* vars trick that was there
before. I missed in my earlier change that nix/local.mk was referring
LIBGCRYPT_* vars directly.

And, instead of CXXFLAGS, CPPFLAGS is used since the latter is probably
more correct as this is used for include dirs, therefore using
preprocessor flags.

Tested with ./configure LDFLAGS="-Wl,--as-needed" --with-libgcrypt-prefix=... combinations.

* config-daemon.ac: Set ‘LIBGCRYPT_CPPFLAGS’ instead of
‘LIBGCRYPT_CXXFLAGS’.  Set ‘LIBGCRYPT_LIBS’ in addition to
‘LIBGCRYPT_LDFLAGS’.  Save and restore ‘CPPFLAGS’, ‘LDFLAGS’, and ‘LIBS’
around test.
* nix/local.mk (libutil_a_CPPFLAGS): Add $(LIBGCRYPT_CPPFLAGS).
(libstore_a_CXXFLAGS): Remove $(LIBGCRYPT_CFLAGS).
(guix_daemon_LDFLAGS): New variable.

Change-Id: Iadb10e1994c9a78e2927847af2cfe5e096fbb2a8
Signed-off-by: Ludovic Courtès <ludo@gnu.org>

nix/local.mk

@@ -69,7 +69,8 @@ libutil_headers =				\
 libutil_a_CPPFLAGS =				\
   -I$(top_builddir)/nix				\
   -I$(top_srcdir)/%D%/libutil			\
-  $(libformat_a_CPPFLAGS)
+  $(libformat_a_CPPFLAGS)			\
+  $(LIBGCRYPT_CPPFLAGS)
 
 libstore_a_SOURCES =				\
   %D%/libstore/gc.cc				\
@@ -109,7 +110,7 @@ libstore_a_CPPFLAGS =				\
   -DDEFAULT_CHROOT_DIRS="\"\""
 
 libstore_a_CXXFLAGS = $(AM_CXXFLAGS)		\
-  $(SQLITE3_CFLAGS) $(LIBGCRYPT_CFLAGS)
+  $(SQLITE3_CFLAGS)
 
 bin_PROGRAMS = guix-daemon
 
@@ -122,6 +123,9 @@ guix_daemon_CPPFLAGS =				\
   $(libutil_a_CPPFLAGS)				\
   -I$(top_srcdir)/%D%/libstore
 
+guix_daemon_LDFLAGS = 				\
+  $(LIBGCRYPT_LDFLAGS)
+
 guix_daemon_LDADD =				\
   libstore.a libutil.a libformat.a -lz		\
   $(SQLITE3_LIBS) $(LIBGCRYPT_LIBS)