eio: Add weak symbol

Summary:
Add definition for EAPI_WEAK because this macro will be needed when we
change how Eolian generates import/export symbols for the Eio library.

=  The Rationale =

This patch is from a series of patches to rename EAPI symbols to
specific library DSOs.

EAPI was designed to be able to pass
`__attribute__ ((visibility ("default")))` for symbols with
GCC, which would mean that even if -fvisibility=hidden was used
when compiling the library, the needed symbols would get exported.

MSVC __almost__ works like GCC (or mingw) in which you can
declare everything as export and it will just work (slower, but
it will work). But there's a caveat: global variables will not
work the same way for MSVC, but works for mingw and GCC.

For global variables (as opposed to functions), MSVC requires
correct DSO visibility for MSVC: instead of declaring a symbol as
export for everything, you need to declare it as import when
importing from another DSO and export when defining it locally.

With current EAPI definitions, we get the following example
working in mingw and MSVC (observe it doesn't define any global
variables as exported symbols).

Example 1:
dll1:
```
EAPI void foo(void);

EAPI void bar()
{
  foo();
}
```
dll2:
```
EAPI void foo()
{
  printf ("foo\n");
}
```

This works fine with API defined as __declspec(dllexport) in both
cases and for gcc defining as
`__atttribute__((visibility("default")))`.

However, the following:
Example 2:

dll1:

```
EAPI extern int foo;
EAPI void foobar(void);

EAPI void bar()
{
  foo = 5;
  foobar();
}
```

dll2:

```
EAPI int foo = 0;
EAPI void foobar()
{
  printf ("foo %d\n", foo);
}
```

This will work on mingw but will not work for MSVC. And that's why
LIBAPI is the only solution that works for MSVC.

Co-authored-by: João Paulo Taylor Ienczak Zanette <jpaulotiz@gmail.com>
Co-authored-by: Lucas Cavalcante de Sousa <lucks.sousa@gmail.com>
Co-authored-by: Ricardo Campos <ricardo.campos@expertise.dev>

Reviewers: jptiz, lucas, vtorri, woohyun

Reviewed By: vtorri

Subscribers: cedric, #reviewers, #committers

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D12201

src/lib/eio/Eio.h

@@ -36,6 +36,10 @@
 # undef EAPI
 #endif
 
+#ifdef EAPI_WEAK
+# undef EAPI_WEAK
+#endif
+
 #ifdef _WIN32
 # ifdef EFL_BUILD
 #  ifdef DLL_EXPORT
@@ -46,15 +50,19 @@
 # else
 #  define EAPI __declspec(dllimport)
 # endif
+# define EAPI_WEAK
 #else
 # ifdef __GNUC__
 #  if __GNUC__ >= 4
 #   define EAPI __attribute__ ((visibility("default")))
+#   define EAPI_WEAK __attribute__ ((weak))
 #  else
 #   define EAPI
+#   define EAPI_WEAK
 #  endif
 # else
 #  define EAPI
+#  define EAPI_WEAK
 # endif
 #endif