build: Introduce SECP256K1_STATIC macro for Windows users

It is a non-Libtool-specific way to explicitly specify the user's
intention to consume a static `libseck256k1`.

This change allows to get rid of MSVC linker warnings LNK4217 and
LNK4286. Also, it makes possible to merge the `SECP256K1_API` and
`SECP256K1_API_VAR` into one.

src/CMakeLists.txt

@@ -24,6 +24,7 @@ endif()
 # This matches libtool's usage of DLL_EXPORT
 if(WIN32)
   set_target_properties(secp256k1 PROPERTIES DEFINE_SYMBOL "DLL_EXPORT")
+  target_compile_definitions(secp256k1 INTERFACE $<$<NOT:$<BOOL:${BUILD_SHARED_LIBS}>>:SECP256K1_STATIC>)
 endif()
 
 # Object libs don't know if they're being built for a shared or static lib.