build: Pass the appropriate flags when building `libstore.a'.

* daemon.am (libstore_a_CFLAGS): Rename to...
  (libstore_a_CXXFLAGS): ... this.

daemon.am

@@ -115,7 +115,7 @@ libstore_a_CPPFLAGS =				\
   -DNIX_BIN_DIR=\"$(bindir)\"			\
   -DOPENSSL_PATH="\"openssl\""
 
-libstore_a_CFLAGS =				\
+libstore_a_CXXFLAGS =				\
   $(SQLITE3_CFLAGS) $(LIBGCRYPT_CFLAGS)
 
 bin_PROGRAMS = guix-daemon