cmake: add support for "unity" builds

Aka "jumbo" or "amalgamation" builds. It means to compile all sources
per target as a single C source. This is experimental.

You can enable it by passing `-DCMAKE_UNITY_BUILD=ON` to cmake.
It requires CMake 3.16 or newer.

It makes builds (much) faster, allows for better optimizations and tends
to promote less ambiguous code.

Also add a new AppVeyor CI job and convert an existing one to use
"unity" mode (one MSVC, one MinGW), and enable it for one macOS CI job.

Fix related issues:
- add missing include guard to `easy_lock.h`.
- rename static variables and functions (and a macro) with names reused
  across sources, or shadowed by local variables.
- add an `#undef` after use.
- add a missing `#undef` before use.
- move internal definitions from `ftp.h` to `ftp.c`.
- `curl_memory.h` fixes to make it work when included repeatedly.
- stop building/linking curlx bits twice for a static-mode curl tool.
  These caused doubly defined symbols in unity builds.
- silence missing extern declarations compiler warning for ` _CRT_glob`.
- fix extern declarations for `tool_freq` and `tool_isVistaOrGreater`.
- fix colliding static symbols in debug mode: `debugtime()` and
  `statename`.
- rename `ssl_backend_data` structure to unique names for each
  TLS-backend, along with the `ssl_connect_data` struct member
  referencing them. This required adding casts for each access.
- add workaround for missing `[P]UNICODE_STRING` types in certain Windows
  builds when compiling `lib/ldap.c`. To support "unity" builds, we had
  to enable `SCHANNEL_USE_BLACKLISTS` for Schannel (a Windows
  `schannel.h` option) _globally_. This caused an indirect inclusion of
  Windows `schannel.h` from `ldap.c` via `winldap.h` to have it enabled
  as well. This requires `[P]UNICODE_STRING` types, which is apperantly
  not defined automatically (as seen with both MSVS and mingw-w64).
  This patch includes `<subauth.h>` to fix it.
  Ref: https://github.com/curl/curl/runs/13987772013
  Ref: https://dev.azure.com/daniel0244/curl/_build/results?buildId=15827&view=logs&jobId=2c9f582d-e278-56b6-4354-f38a4d851906&j=2c9f582d-e278-56b6-4354-f38a4d851906&t=90509b00-34fa-5a81-35d7-5ed9569d331c
- tweak unity builds to compile `lib/memdebug.c` separately in memory
  trace builds to avoid PP confusion.
- force-disable unity for test programs.
- do not compile and link libcurl sources to libtests _twice_ when libcurl
  is built in static mode.

KNOWN ISSUES:
- running tests with unity builds may fail in cases.
- some build configurations/env may not compile in unity mode. E.g.:
  https://ci.appveyor.com/project/curlorg/curl/builds/47230972/job/51wfesgnfuauwl8q#L250

Ref: https://github.com/libssh2/libssh2/issues/1034
Ref: https://cmake.org/cmake/help/latest/prop_tgt/UNITY_BUILD.html
Ref: https://en.wikipedia.org/wiki/Unity_build

Closes #11095

src/CMakeLists.txt

@@ -58,13 +58,17 @@ transform_makefile_inc("Makefile.inc" "${CMAKE_CURRENT_BINARY_DIR}/Makefile.inc.
 include(${CMAKE_CURRENT_BINARY_DIR}/Makefile.inc.cmake)
 
 if(WIN32)
-  list(APPEND CURL_FILES curl.rc)
+  list(APPEND CURL_CFILES curl.rc)
+endif()
+
+# CURL_CFILES, CURLX_CFILES, CURL_HFILES come from Makefile.inc
+if(NOT BUILD_SHARED_LIBS)
+  set(CURLX_CFILES ../lib/dynbuf.c)
 endif()
 
-# CURL_FILES comes from Makefile.inc
 add_executable(
   ${EXE_NAME}
-  ${CURL_FILES}
+  ${CURL_CFILES} ${CURLX_CFILES} ${CURL_HFILES}
   )
 
 add_executable(

src/tool_findfile.c

@@ -49,7 +49,7 @@ struct finder {
 
 /* The order of the variables below is important, as the index number is used
    in the findfile() function */
-static const struct finder list[] = {
+static const struct finder conf_list[] = {
   { "CURL_HOME", NULL, FALSE },
   { "XDG_CONFIG_HOME", NULL, FALSE }, /* index == 1, used in the code */
   { "HOME", NULL, FALSE },
@@ -109,8 +109,8 @@ char *findfile(const char *fname, int dotscore)
   if(!fname[0])
     return NULL;
 
-  for(i = 0; list[i].env; i++) {
-    char *home = curl_getenv(list[i].env);
+  for(i = 0; conf_list[i].env; i++) {
+    char *home = curl_getenv(conf_list[i].env);
     if(home) {
       char *path;
       const char *filename = fname;
@@ -120,14 +120,14 @@ char *findfile(const char *fname, int dotscore)
         curl_free(home);
         continue;
       }
-      if(list[i].append) {
-        char *c = curl_maprintf("%s%s", home, list[i].append);
+      if(conf_list[i].append) {
+        char *c = curl_maprintf("%s%s", home, conf_list[i].append);
         curl_free(home);
         if(!c)
           return NULL;
         home = c;
       }
-      if(list[i].withoutdot) {
+      if(conf_list[i].withoutdot) {
         if(!dotscore || xdg) {
           /* this is not looking for .curlrc, or the XDG_CONFIG_HOME was
              defined so we skip the extended check */

src/tool_main.c

@@ -78,6 +78,7 @@ int vms_show = 0;
  * when command-line argument globbing is enabled under the MSYS shell, so turn
  * it off.
  */
+extern int _CRT_glob;
 int _CRT_glob = 0;
 #endif /* __MINGW32__ */
 

src/tool_setup.h

@@ -73,4 +73,10 @@ extern FILE *tool_stderr;
 #  include "tool_strdup.h"
 #endif
 
+#if defined(WIN32) && !defined(MSDOS)
+/* set in win32_init() */
+extern LARGE_INTEGER tool_freq;
+extern bool tool_isVistaOrGreater;
+#endif
+
 #endif /* HEADER_CURL_TOOL_SETUP_H */

src/tool_util.c

@@ -33,10 +33,6 @@
 
 #if defined(WIN32) && !defined(MSDOS)
 
-/* set in win32_init() */
-extern LARGE_INTEGER tool_freq;
-extern bool tool_isVistaOrGreater;
-
 /* In case of bug fix this function has a counterpart in timeval.c */
 struct timeval tvnow(void)
 {