Add `IREE_ENABLE_WERROR_FLAG` CMake option. (#16121)

> Agree with @ Jakub Kuderski : it isn't a great default for people
using the project on random systems.
> By Stella.

Simple change to add option to disable `-Werror` flag.

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 5617704..39d03fd 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -439,6 +439,7 @@
 option(IREE_ENABLE_SPLIT_DWARF "Enable gsplit-dwarf for debug information if the platform supports it" OFF)
 option(IREE_ENABLE_THIN_ARCHIVES "Enables thin ar archives (elf systems only). Disable for released static archives" OFF)
 option(IREE_LINK_COMPILER_SHARED_LIBRARY "Links IREE tools using the compiler compiled into a shared library" ON)
+option(IREE_ENABLE_WERROR_FLAG "Enable `-Werror` flag, treat error as warning" ON)
 
 # STREQUAL feels wrong here - we don't care about the exact true-value used,
 # ON or TRUE or something else. But we haven't been able to think of a less bad

build_tools/cmake/iree_copts.cmake

diff --git a/build_tools/cmake/iree_copts.cmake b/build_tools/cmake/iree_copts.cmake
index 038e9b3..5b36433 100644
--- a/build_tools/cmake/iree_copts.cmake
+++ b/build_tools/cmake/iree_copts.cmake
@@ -151,7 +151,7 @@
   # internally is very useful when importing. If you feel that some of these
   # should be different (especially more strict), please raise an issue!
   CLANG
-    "-Werror"
+    "$<$<BOOL:${IREE_ENABLE_WERROR_FLAG}>:-Werror>"
     "-Wall"
 
     "-Wno-error=deprecated-declarations"  # Want to see them but defaults to error.
@@ -217,7 +217,7 @@
 
   GCC
     "-Wall"
-    "-Werror"
+    "$<$<BOOL:${IREE_ENABLE_WERROR_FLAG}>:-Werror>"
     "-Wno-error=deprecated-declarations"  # Want to see them but defaults to error.
 
     "-Wno-address"  # https://github.com/openxla/iree/issues/16016