build: Use SECP256K1_STATICLIB macro instead of warning suppressions

This change allows the user to define the `SECP256K1_STATICLIB` macro
instead of ignoring MSVC linker warnings LNK4217 and LNK4286.

Makefile.am

@@ -153,7 +153,7 @@ endif
 if USE_EXAMPLES
 noinst_PROGRAMS += ecdsa_example
 ecdsa_example_SOURCES = examples/ecdsa.c
-ecdsa_example_CPPFLAGS = -I$(top_srcdir)/include
+ecdsa_example_CPPFLAGS = -I$(top_srcdir)/include -DSECP256K1_STATICLIB
 ecdsa_example_LDADD = libsecp256k1.la
 ecdsa_example_LDFLAGS = -static
 if BUILD_WINDOWS
@@ -163,7 +163,7 @@ TESTS += ecdsa_example
 if ENABLE_MODULE_ECDH
 noinst_PROGRAMS += ecdh_example
 ecdh_example_SOURCES = examples/ecdh.c
-ecdh_example_CPPFLAGS = -I$(top_srcdir)/include
+ecdh_example_CPPFLAGS = -I$(top_srcdir)/include -DSECP256K1_STATICLIB
 ecdh_example_LDADD = libsecp256k1.la
 ecdh_example_LDFLAGS = -static
 if BUILD_WINDOWS
@@ -174,7 +174,7 @@ endif
 if ENABLE_MODULE_SCHNORRSIG
 noinst_PROGRAMS += schnorr_example
 schnorr_example_SOURCES = examples/schnorr.c
-schnorr_example_CPPFLAGS = -I$(top_srcdir)/include
+schnorr_example_CPPFLAGS = -I$(top_srcdir)/include -DSECP256K1_STATICLIB
 schnorr_example_LDADD = libsecp256k1.la
 schnorr_example_LDFLAGS = -static
 if BUILD_WINDOWS

examples/CMakeLists.txt

@@ -7,7 +7,7 @@ target_link_libraries(example INTERFACE
   $<$<PLATFORM_ID:Windows>:bcrypt>
 )
 if(NOT BUILD_SHARED_LIBS AND MSVC)
-  target_link_options(example INTERFACE /IGNORE:4217)
+  target_compile_definitions(example INTERFACE SECP256K1_STATICLIB)
 endif()
 
 add_executable(ecdsa_example ecdsa.c)

include/secp256k1.h

@@ -140,7 +140,7 @@ typedef int (*secp256k1_nonce_function)(
 #   define SECP256K1_API            __declspec (dllexport)
 #   define SECP256K1_API_VAR extern __declspec (dllexport)
 #  endif
-# elif defined _MSC_VER
+# elif defined(_MSC_VER) && !defined(SECP256K1_STATICLIB)
 #  define SECP256K1_API
 #  define SECP256K1_API_VAR  extern __declspec (dllimport)
 # elif defined DLL_EXPORT