build: tidy up and improve versioned-symbols options

- autotools: add support for custom prefix.
  Usage: `--enable-versioned-symbols=MYPREFIX_`
  Catching up with cmake.

- add default versioned prefix for Rustls builds.

- delete `HIDDEN` entry from version script `lib/libcurl.vers.in`.
  It referred to symbol names that never existed in libcurl.

- cmake: use `lib/libcurl.vers.in` as a template instead of generating
  it from scratch. Adapt `./configure` accordingly, and add comments
  to keep hard-wired soname in sync with `lib/Makefile.soname`.

- autotools: delete Schannel and Secure Transport version prefixes.
  Windows and macOS don't support the versioned symbols option.

Follow-up to 7b14449790 #14378
Closes #14818

lib/CMakeLists.txt

@@ -238,15 +238,18 @@ if(BUILD_SHARED_LIBS)
         set(CURL_LIBCURL_VERSIONED_SYMBOLS_PREFIX "WOLFSSL_")
       elseif(CURL_USE_GNUTLS)
         set(CURL_LIBCURL_VERSIONED_SYMBOLS_PREFIX "GNUTLS_")
+      elseif(CURL_USE_RUSTLS)
+        set(CURL_LIBCURL_VERSIONED_SYMBOLS_PREFIX "RUSTLS_")
       endif()
     endif()
-    file(WRITE "${CMAKE_CURRENT_BINARY_DIR}/libcurl.vers" "
-      HIDDEN {};
-      CURL_${CURL_LIBCURL_VERSIONED_SYMBOLS_PREFIX}${_cmakesoname}
-      {
-        global: curl_*;
-        local: *;
-      };")
+    # Generate version script for the linker, for versioned symbols.
+    # Consumed variables:
+    #   CURL_LIBCURL_VERSIONED_SYMBOLS_PREFIX
+    #   CURL_LIBCURL_VERSIONED_SYMBOLS_SONAME
+    set(CURL_LIBCURL_VERSIONED_SYMBOLS_SONAME ${_cmakesoname})
+    configure_file(
+      "${CMAKE_CURRENT_SOURCE_DIR}/libcurl.vers.in"
+      "${CMAKE_CURRENT_BINARY_DIR}/libcurl.vers" @ONLY)
     include(CheckCSourceCompiles)
     set(CMAKE_REQUIRED_LINK_OPTIONS "-Wl,--version-script=${CMAKE_CURRENT_BINARY_DIR}/libcurl.vers")
     check_c_source_compiles("int main(void) { return 0; }" HAVE_VERSIONED_SYMBOLS)

lib/Makefile.soname

@@ -22,6 +22,7 @@
 #
 ###########################################################################
 
+# Keep in sync with CURL_LIBCURL_VERSIONED_SYMBOLS_SONAME in configure.ac
 VERSIONCHANGE=12
 VERSIONADD=0
 VERSIONDEL=8

lib/libcurl.vers.in

@@ -1,12 +1,4 @@
-HIDDEN
-{
-  local:
-    __*;
-    _rest*;
-    _save*;
-};
-
-CURL_@CURL_LT_SHLIB_VERSIONED_FLAVOUR@4
+CURL_@CURL_LIBCURL_VERSIONED_SYMBOLS_PREFIX@@CURL_LIBCURL_VERSIONED_SYMBOLS_SONAME@
 {
   global: curl_*;
   local: *;