build: add -no-undefined to libtool LDFLAGS

Instruct libtool to not allow undefined symbols when linking a shared
library.

See:
https://autotools.io/libtool/windows.html
https://www.gnu.org/software/libtool/manual/libtool.html#LT_005fINIT
https://www.gnu.org/software/gnulib/manual/html_node/Libtool-and-Windows.html

Makefile.am

@@ -74,6 +74,7 @@ endif
 libsecp256k1_la_SOURCES = src/secp256k1.c
 libsecp256k1_la_CPPFLAGS = -I$(top_srcdir)/include -I$(top_srcdir)/src $(SECP_INCLUDES)
 libsecp256k1_la_LIBADD = $(SECP_LIBS) $(COMMON_LIB)
+libsecp256k1_la_LDFLAGS = -no-undefined
 
 if VALGRIND_ENABLED
 libsecp256k1_la_CPPFLAGS += -DVALGRIND