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.gtest.include

@@ -21,6 +21,7 @@ hush_gtest_CXXFLAGS = $(AM_CXXFLAGS) $(PIE_FLAGS)
 # $(LIBBITCOIN_UNIVALUE) so univalue was never linked, and omitted LIBHUSH/LIBRANDOMX/libcc).
 hush_gtest_LDADD = -lgtest -lgmock \
   $(LIBBITCOIN_SERVER) \
+  $(LIBCURL) \
   $(LIBBITCOIN_COMMON) \
   $(LIBUNIVALUE) \
   $(LIBBITCOIN_UTIL) \