Fix automake -lcurl portability lint in Makefile

LIBBITCOIN_SERVER was fed into both EXTRA_LIBRARIES (a list of buildable
library files) and several _LDADD link lines. Embedding the -lcurl linker
flag inside it made automake reject it in the EXTRA_LIBRARIES context
("'-lcurl' is not a standard library name"). Make LIBBITCOIN_SERVER a pure
file and route -lcurl through its own LIBCURL variable, added to the
dragonxd, hush-gtest, and test_bitcoin link lines after libbitcoin_server.a
(whose objects reference curl symbols) so static link order stays correct.

Verified with a clean Windows cross-build (-DCURL_STATICLIB) and a native
Linux build: both link cleanly and the automake lint is gone.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/Makefile.am

@@ -39,15 +39,22 @@ BITCOIN_INCLUDES += -I$(srcdir)/univalue/include
 BITCOIN_INCLUDES += -I$(srcdir)/leveldb/include
 
 if TARGET_WINDOWS
-LIBBITCOIN_SERVER=libbitcoin_server.a -lcurl
+LIBBITCOIN_SERVER=libbitcoin_server.a
 endif
 if TARGET_DARWIN
-LIBBITCOIN_SERVER=libbitcoin_server.a -lcurl
+LIBBITCOIN_SERVER=libbitcoin_server.a
 endif
 if TARGET_LINUX
-LIBBITCOIN_SERVER=libbitcoin_server.a -lcurl
+LIBBITCOIN_SERVER=libbitcoin_server.a
 endif
 
+# libcurl is a linker flag, not a buildable library file. It must NOT live inside
+# LIBBITCOIN_SERVER, which is also fed into EXTRA_LIBRARIES (a list of files automake
+# builds) where a -l flag is illegal and triggers a portability error. Keep it as its
+# own variable, added to each binary's _LDADD after libbitcoin_server.a (whose objects
+# reference curl symbols) so static link order stays correct.
+LIBCURL = -lcurl
+
 LIBBITCOIN_WALLET=libbitcoin_wallet.a
 LIBBITCOIN_COMMON=libbitcoin_common.a
 LIBBITCOIN_CLI=libbitcoin_cli.a
@@ -464,6 +471,7 @@ endif
 
 dragonxd_LDADD = \
   $(LIBBITCOIN_SERVER) \
+  $(LIBCURL) \
   $(LIBBITCOIN_COMMON) \
   $(LIBUNIVALUE) \
   $(LIBBITCOIN_UTIL) \