build: untangle CURLDEBUG and DEBUGBUILD macros

`CURLDEBUG` is meant to enable memory tracking, but in a bunch of cases,
it was protecting debug features that were supposed to be guarded with
`DEBUGBUILD`.

Replace these uses with `DEBUGBUILD`.

This leaves `CURLDEBUG` uses solely for its intended  purpose: to enable
the memory tracking debug feature.

Also:
- autotools: rely on `DEBUGBUILD` to enable `checksrc`.
  Instead of `CURLDEBUG`, which worked in most cases because debug
  builds enable `CURLDEBUG` by default, but it's not accurate.
- include `lib/easyif.h` instead of keeping a copy of a declaration.
- add CI test jobs for the build issues discovered.

Ref: https://github.com/curl/curl/pull/13694#issuecomment-2120311894
Closes #13718

src/Makefile.am

@@ -145,7 +145,7 @@ checksrc:
 	$(CHECKSRC)(@PERL@ $(top_srcdir)/scripts/checksrc.pl -D$(srcdir) \
 	-W$(srcdir)/tool_hugehelp.c $(srcdir)/*.[ch])
 
-if CURLDEBUG
+if DEBUGBUILD
 # for debug builds, we scan the sources on all regular make invokes
 all-local: checksrc
 endif

src/tool_cfgable.h

@@ -323,7 +323,7 @@ struct GlobalConfig {
   bool styled_output;             /* enable fancy output style detection */
   long ms_per_transfer;           /* start next transfer after (at least) this
                                      many milliseconds */
-#ifdef CURLDEBUG
+#ifdef DEBUGBUILD
   bool test_event_based;
 #endif
   bool parallel;

src/tool_getparam.c

@@ -1825,7 +1825,7 @@ ParameterError getparameter(const char *flag, /* f or -long-flag */
       config->sasl_ir = toggle;
       break;
     case C_TEST_EVENT: /* --test-event */
-#ifdef CURLDEBUG
+#ifdef DEBUGBUILD
       global->test_event_based = toggle;
 #else
       warnf(global, "--test-event is ignored unless a debug build");

src/tool_operate.c

@@ -83,14 +83,12 @@
 #include "tool_progress.h"
 #include "tool_ipfs.h"
 #include "dynbuf.h"
+#ifdef DEBUGBUILD
+#include "easyif.h"  /* for libcurl's debug-only curl_easy_perform_ev() */
+#endif
 
 #include "memdebug.h" /* keep this as LAST include */
 
-#ifdef CURLDEBUG
-/* libcurl's debug builds provide an extra function */
-CURLcode curl_easy_perform_ev(CURL *easy);
-#endif
-
 #ifndef O_BINARY
 /* since O_BINARY as used in bitmasks, setting it to zero makes it usable in
    source code but yet it doesn't ruin anything */
@@ -1315,7 +1313,7 @@ static CURLcode single_transfer(struct GlobalConfig *global,
         my_setopt(curl, CURLOPT_SEEKFUNCTION, tool_seek_cb);
 
         {
-#ifdef CURLDEBUG
+#ifdef DEBUGBUILD
           char *env = getenv("CURL_BUFFERSIZE");
           if(env) {
             long size = strtol(env, NULL, 10);
@@ -1647,7 +1645,7 @@ static CURLcode single_transfer(struct GlobalConfig *global,
            *  must do the same thing as classic:
            *    --cert <filename>:<password> --cert-type p12
            *  but is designed to test blob */
-#if defined(CURLDEBUG) || defined(DEBUGBUILD)
+#ifdef DEBUGBUILD
           if(config->cert && (strlen(config->cert) > 8) &&
              (memcmp(config->cert, "loadmem=",8) == 0)) {
             FILE *fInCert = fopen(config->cert + 8, "rb");
@@ -1690,7 +1688,7 @@ static CURLcode single_transfer(struct GlobalConfig *global,
                         config->proxy_cert_type);
 
 
-#if defined(CURLDEBUG) || defined(DEBUGBUILD)
+#ifdef DEBUGBUILD
           if(config->key && (strlen(config->key) > 8) &&
              (memcmp(config->key, "loadmem=",8) == 0)) {
             FILE *fInCert = fopen(config->key + 8, "rb");
@@ -2484,7 +2482,7 @@ static CURLcode serial_transfers(struct GlobalConfig *global,
         break;
     }
     start = tvnow();
-#ifdef CURLDEBUG
+#ifdef DEBUGBUILD
     if(global->test_event_based)
       result = curl_easy_perform_ev(per->curl);
     else