Refactor Base64 functions, encode/decode using OpenSSL's EVP interface

Our own implementation works fine, however we should use OpenSSL's one since we already link to the library.

Base64Decode() and Base64Encode() return the required buffer size when "dst" is NULL.

This allows to efficiently allocate a buffer, without wasting memory or risking an overflow.

Base64FromBin() and Base64ToBin() perform all steps, returning a heap-allocated buffer with the data in it.

src/Mayaqua/CMakeLists.txt

@@ -57,9 +57,16 @@ if(UNIX)
   # In some cases libiconv is not included in libc
   find_library(LIB_ICONV iconv)
 
+  find_library(LIB_M m)
   find_library(LIB_RT rt)
 
-  target_link_libraries(mayaqua PRIVATE Threads::Threads)
+  target_link_libraries(mayaqua
+    PRIVATE
+      Threads::Threads
+      $<$<BOOL:${LIB_ICONV}>:${LIB_ICONV}>
+      $<$<BOOL:${LIB_M}>:${LIB_M}>
+      $<$<BOOL:${LIB_RT}>:${LIB_RT}>
+  )
 
   if (CMAKE_SYSTEM_PROCESSOR MATCHES "^(armv7l|aarch64|s390x)$" OR NOT HAVE_SYS_AUXV OR SKIP_CPU_FEATURES)
     add_definitions(-DSKIP_CPU_FEATURES)
@@ -69,14 +76,6 @@ if(UNIX)
     target_link_libraries(mayaqua PRIVATE cpu_features)
   endif()
 
-  if(LIB_RT)
-    target_link_libraries(mayaqua PRIVATE rt)
-  endif()
-
-  if(LIB_ICONV)
-    target_link_libraries(mayaqua PRIVATE ${LIB_ICONV})
-  endif()
-
   if(${CMAKE_SYSTEM_NAME} STREQUAL "SunOS")
     target_link_libraries(mayaqua PRIVATE nsl socket)
   endif()