build: Add SECP256K1_FORCE_HIDDEN_VISIBILITY

The name should make it clear that this is intended for ELF platforms
only.

Author: real-or-random

include/secp256k1.h

@@ -151,15 +151,30 @@ typedef int (*secp256k1_nonce_function)(
 #  define SECP256K1_API extern __declspec (dllimport)
 # endif
 #endif
+#if !defined(SECP256K1_API)                                                   \
+      && defined(__GNUC__) && (__GNUC__ >= 4) && defined(SECP256K1_BUILD)     \
+      && defined(SECP256K1_FORCE_HIDDEN_VISIBILITY)
+ /* Building libsecp256k1 using GCC or compatible on non-Windows, and hidden
+  * visibility is requested explicitly.
+  *
+  * Forcing hidden visibility can be useful, e.g., when building a static
+  * library which is linked into a shared library, and the latter should not
+  * reexport the libsecp256k1 API. Since this will create an unusable shared
+  * library, SECP256K1_FORCE_HIDDEN_VISIBILITY should only be used when building
+  * a static library. (You want to ./configure with --disable-shared if using
+  * Autotools.)
+  *
+  * While visibility is a concept that applies only to shared libraries, setting
+  * visibility will still make a difference when building a static library: the
+  * visibility settings will be stored in the static library, solely for the
+  * potential case that the static library will be linked into a shared library.
+  * In that case, the stored visibility settings will resurface and be honored
+  * for the shared library. */
+# define SECP256K1_API extern __attribute__ ((visibility ("hidden")))
+#endif
 #ifndef SECP256K1_API
-/* All cases not captured by the Windows-specific logic. */
-# if defined(__GNUC__) && (__GNUC__ >= 4) && defined(SECP256K1_BUILD)
-   /* Building libsecp256k1 using GCC or compatible. */
-#  define SECP256K1_API extern
-# else
-   /* Fall back to standard C's extern. */
-#  define SECP256K1_API extern
-# endif
+  /* Fall back to standard C's extern. */
+# define SECP256K1_API extern
 #endif
 
 /* Warning attributes