crypt-port: Add the bits for compiling with link-time optimization.

GCC 10.2 and LLVM/Clang 10 offer initial support for building
libraries, that are using symbol versioning features, with LTO.

To make use of this with GCC 10.2, the exported versioned symbols
need to be declared explicitly with __attribute__((symver (...))).

LLVM/Clang 10 supports symbol versioning with LTO out of the box
without any changes needed.

Fixes #24.

lib/crypt-port.h

@@ -179,11 +179,29 @@ _crypt_strcpy_or_abort (void *, const size_t, const void *);
 # define _strong_alias(name, aliasname) \
   extern __typeof (name) aliasname __THROW __attribute__ ((alias (#name)))
 
+/* Starting with GCC 10.2, we can use the symver attribute, which also works
+   with link-time optimization enabled.  */
+# if __GNUC__ > 10 || (__GNUC__ == 10 && __GNUC_MINOR__ >= 2)
+
+/* Referencing specific _compatibility_ symbols is still needs inline asm.  */
+# define _symver_ref(extstr, intname, version) \
+  __asm__ (".symver " #intname "," extstr "@" #version)
+
+/* Set the symbol version for EXTNAME, which uses INTNAME as its
+   implementation.  */
+# define symver_set(extstr, intname, version, mode) \
+  extern __typeof (intname) intname __THROW \
+    __attribute__((symver (extstr mode #version)))
+
+# else
+
 /* Set the symbol version for EXTNAME, which uses INTNAME as its
    implementation.  */
 # define symver_set(extstr, intname, version, mode) \
   __asm__ (".symver " #intname "," extstr mode #version)
 
+# endif
+
 #else
 # error "Don't know how to do symbol versioning with this compiler"
 #endif
@@ -239,9 +257,14 @@ _crypt_strcpy_or_abort (void *, const size_t, const void *);
 
 /* Tests may need to _refer_ to compatibility symbols, but should never need
    to _define_ them.  */
-
 #define symver_ref(extstr, intname, version) \
+  _symver_ref(extstr, intname, version)
+
+/* Generic way for referencing specific _compatibility_ symbols.  */
+#ifndef _symver_ref
+#define _symver_ref(extstr, intname, version) \
   symver_set(extstr, intname, version, "@")
+#endif
 
 /* Define configuration macros used during compile-time by the
    GOST R 34.11-2012 "Streebog" hash function.  */