Third-party: Disable -Werror for flatcc build

Apple Clang 21 (Xcode 26) elevates -Wimplicit-int-conversion-on-negation
and -Wunterminated-string-initialization to errors, which breaks the
vendored flatcc build at pprintint.h and grisu3_print.h. The warnings
are benign and the fix belongs upstream in dvidelabs/flatcc; disable
-Werror locally via FLATCC_ALLOW_WERROR=OFF for both the flatcc_ep
external project (host compiler) and the in-tree flatccrt runtime lib.

Authored with Claude.

Author: rascani

third-party/CMakeLists.txt

@@ -91,6 +91,7 @@ ExternalProject_Add(
              -DFLATCC_TEST=OFF
              -DFLATCC_REFLECTION=OFF
              -DFLATCC_DEBUG_CLANG_SANITIZE=OFF
+             -DFLATCC_ALLOW_WERROR=OFF
              -DFLATCC_INSTALL=ON
              -DCMAKE_POLICY_VERSION_MINIMUM=3.5
              -DCMAKE_INSTALL_PREFIX:PATH=<INSTALL_DIR>
@@ -116,6 +117,7 @@ set(FLATCC_RTONLY ON CACHE BOOL "")
 set(FLATCC_TEST OFF CACHE BOOL "")
 set(FLATCC_REFLECTION OFF CACHE BOOL "")
 set(FLATCC_DEBUG_CLANG_SANITIZE OFF CACHE BOOL "")
+set(FLATCC_ALLOW_WERROR OFF CACHE BOOL "")
 set(FLATCC_INSTALL OFF CACHE BOOL "")
 add_subdirectory(flatcc)
 # Unfortunately flatcc writes libs directly in to the source tree [1]. So to