build: set -Wno-format-signedness

Explicitly disable these warnings to allow using `-Weverything`.

There are around 600 of them across the codebase.

Silencing them has some drawbacks:
- enums (`CURLcode` mostly) would have to be cast to int to avoid
  different signedness depending on C compiler.
  (llvm/gcc: unsigned, MSVC/clang-cl: signed by default)
- hex masks need casts to unsigned to avoid the warning.
- fixing remaining warnings is annoying without fixing the above.
- without fixing all warnings the option cannot be enabled, to keep
  the codebase warning free.

Ref: #18343 (silenced all warnings, but without the enum cast)
Follow-up to 92f215fea1 #18477
Closes #19907

CMake/PickyWarnings.cmake

@@ -232,6 +232,12 @@ if(PICKY_COMPILER)
           -Wcast-function-type-strict      # clang 16.0            appleclang 16.0
         )
       endif()
+      if((CMAKE_C_COMPILER_ID STREQUAL "Clang"      AND CMAKE_C_COMPILER_VERSION VERSION_GREATER_EQUAL 19.1) OR
+         (CMAKE_C_COMPILER_ID STREQUAL "AppleClang" AND CMAKE_C_COMPILER_VERSION VERSION_GREATER_EQUAL 17.0))
+        list(APPEND _picky_enable
+          -Wno-format-signedness           # clang 19.1  gcc  5.1  appleclang 17.0  # In clang-cl enums are signed ints by default
+        )
+      endif()
       if(CMAKE_C_COMPILER_ID STREQUAL "Clang"      AND CMAKE_C_COMPILER_VERSION VERSION_GREATER_EQUAL 21.1)
         list(APPEND _picky_enable
           -Warray-compare                  # clang 20.1  gcc 12.0  appleclang ?
@@ -282,6 +288,7 @@ if(PICKY_COMPILER)
       if(CMAKE_C_COMPILER_VERSION VERSION_GREATER_EQUAL 5.0)
         list(APPEND _picky_enable
           -Warray-bounds=2                 # clang  3.0  gcc  5.0 (clang default: -Warray-bounds)
+          -Wno-format-signedness           # clang 19.1  gcc  5.1  appleclang 17.0
         )
       endif()
       if(CMAKE_C_COMPILER_VERSION VERSION_GREATER_EQUAL 6.0)

m4/curl-compilers.m4

@@ -936,6 +936,10 @@ AC_DEFUN([CURL_SET_COMPILER_WARNING_OPTS], [
           if test "$compiler_num" -ge "1700"; then
             CURL_ADD_COMPILER_WARNINGS([tmp_CFLAGS], [cast-function-type-strict])  # with Apple clang it requires 16.0 or above
           fi
+          dnl clang 19 or later
+          if test "$compiler_num" -ge "1901"; then
+            tmp_CFLAGS="$tmp_CFLAGS -Wno-format-signedness"
+          fi
           dnl clang 20 or later
           if test "$compiler_num" -ge "2001"; then
             CURL_ADD_COMPILER_WARNINGS([tmp_CFLAGS], [array-compare])
@@ -1115,6 +1119,7 @@ AC_DEFUN([CURL_SET_COMPILER_WARNING_OPTS], [
           dnl Only gcc 5 or later
           if test "$compiler_num" -ge "500"; then
             tmp_CFLAGS="$tmp_CFLAGS -Warray-bounds=2"
+            tmp_CFLAGS="$tmp_CFLAGS -Wno-format-signedness"
           fi
           #
           dnl Only gcc 6 or later