Merge bitcoin-core/secp256k1#1022: build: Windows DLL additions

c0cd7de build: add -no-undefined to libtool LDFLAGS (fanquake)
fe32a79 build: pass win32-dll to LT_INIT (fanquake)

Pull request description:

  This takes care of two of the outstanding issues in #923. One being initializing libtool with `win32-dll` and the other being the addition of `-no-undefined` to the libtool LDFLAGS. See each commit for more details.

  Builders cross-compiling for Windows (including Core) will no-longer see:
  ```bash
  libtool: warning: undefined symbols not allowed in x86_64-w64-mingw32 shared libraries; building static only
  ```

  I'm planning on making some related changes downstream.

ACKs for top commit:
  sipa:
    utACK c0cd7de. We indeed have done the work to propertly mark exported symbols, and AFAIK have no imported symbols apart from standard library ones.
  real-or-random:
    ACK c0cd7de
  hebasto:
    ACK c0cd7de

Tree-SHA512: 6756bc88ac439a27117a1341d82a801cef70354a9e7a563592ab3ac7298fbefdaa0a2c410ea3fba8953d53f254c449dc491069f30468db12791027a65dd02f80

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

configure.ac

@@ -8,7 +8,7 @@ AH_TOP([#define LIBSECP256K1_CONFIG_H])
 AH_BOTTOM([#endif /*LIBSECP256K1_CONFIG_H*/])
 AM_INIT_AUTOMAKE([foreign subdir-objects])
 
-LT_INIT
+LT_INIT([win32-dll])
 
 # Make the compilation flags quiet unless V=1 is used.
 m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES([yes])])