mk-unity.pl: #include, and not concatenate input headers

When using `-D_CURL_TESTS_CONCAT=ON` with CMake, do not concatenate
`first.h` (or any future header) into the output C file, but `#include`
it instead. This is to play nice with compilers and analyzers which may
apply different checker rules on logic found in headers, vs. the input
source file. As seen for example with `-Wunused-macro` enabled in CI.
After this patch concatenated sources behave closer to regular C
sources.

Also:
- first.h: drop some `-Wunused-macro` silencers that became redundant
  with this patch.

Follow-up to 47f411c6d8 #21554
Follow-up to 39542f0993 #20667

Closes #21656

scripts/mk-unity.pl

@@ -65,7 +65,7 @@ foreach my $src (@ARGV) {
 
 sub include($@) {
     my $filename = shift;
-    if($concat) {
+    if($concat && $filename =~ /([a-z0-9_]+)\.c$/) {
         if(! -f $filename) {
             foreach my $path (@incpath) {
                 my $fullfn = $path . "/" . $filename;