cmake: don't set default visibility for static builds

define SECP256K1_CMAKE_SHARED_BUILD for all CMake shared lib builds, not just
Windows. This allows us to only set default symbol visibility for shared lib builds.

For static libs built with CMake, or Autotools builds for Windows, static
builds will prefer to hide symbols by default. Autotools does not have the
ability to set a define for non-Windows shared builds, so the best we can do
there is guess. SECP256K1_NO_EXPORT_SYMBOLS can be defined as an escape-hatch
in case a non-Windows CMake builder wants default visibility.

Author: theuni

configure.ac

@@ -45,6 +45,7 @@ AS_UNSET(ac_cv_prog_AR)
 AS_UNSET(ac_cv_prog_ac_ct_AR)
 LT_INIT([win32-dll])
 
+SECP_CONFIG_DEFINES="$SECP_CONFIG_DEFINES -DSECP256K1_AUTOTOOLS_BUILD=1"
 build_windows=no
 
 case $host_os in

include/secp256k1.h

@@ -129,10 +129,11 @@ typedef int (*secp256k1_nonce_function)(
    * Attributes" in the GCC manual and the recommendations in
    * https://gcc.gnu.org/wiki/Visibility. */
 # if defined(SECP256K1_BUILD)
-#  if defined(DLL_EXPORT) || defined(SECP256K1_DLL_EXPORT)
+#  if defined(DLL_EXPORT) || defined(SECP256K1_CMAKE_SHARED_BUILD) || defined(SECP256K1_DLL_EXPORT)
     /* Building libsecp256k1 as a DLL.
      * 1. If using Libtool, it defines DLL_EXPORT automatically.
-     * 2. In other cases, SECP256K1_DLL_EXPORT must be defined. */
+     * 2. If using CMake, it defines SECP256K1_CMAKE_SHARED_BUILD automatically.
+     * 3. In other cases, SECP256K1_DLL_EXPORT must be defined. */
 #   define SECP256K1_API extern __declspec (dllexport)
 #  else
     /* Building libsecp256k1 as a static library on Windows.
@@ -155,11 +156,16 @@ typedef int (*secp256k1_nonce_function)(
 /* 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 __attribute__ ((visibility ("default")))
-# else
+#  if !defined(SECP256K1_NO_EXPORT_SYMBOLS)
+#   if defined(SECP256K1_CMAKE_SHARED_BUILD) || defined(SECP256K1_AUTOTOOLS_BUILD)
+#    define SECP256K1_API extern __attribute__ ((visibility ("default")))
+#   endif
+#  endif
+# endif
+#endif
+#ifndef SECP256K1_API
    /* Fall back to standard C's extern. */
 #  define SECP256K1_API extern
-# endif
 #endif
 
 /* Warning attributes

src/CMakeLists.txt

@@ -20,9 +20,9 @@ if(SECP256K1_ASM STREQUAL "arm32")
   target_link_libraries(secp256k1_asm INTERFACE secp256k1_asm_arm)
 endif()
 
+# Define our export symbol only for shared libs.
+set_target_properties(secp256k1 PROPERTIES DEFINE_SYMBOL SECP256K1_CMAKE_SHARED_BUILD)
 if(WIN32)
-  # Define our export symbol only for shared libs.
-  set_target_properties(secp256k1 PROPERTIES DEFINE_SYMBOL SECP256K1_DLL_EXPORT)
   target_compile_definitions(secp256k1 INTERFACE $<$<NOT:$<BOOL:${BUILD_SHARED_LIBS}>>:SECP256K1_STATIC>)
 endif()