Add IREE_ENABLE_WERROR_FLAG CMake option. (iree-org#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

@@ -439,6 +439,7 @@ option(IREE_ENABLE_UBSAN "Enable undefined behavior sanitizer" OFF)
 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

@@ -151,7 +151,7 @@ iree_select_compiler_opts(IREE_DEFAULT_COPTS
   # 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 @@ iree_select_compiler_opts(IREE_DEFAULT_COPTS
 
   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