if editing the meson.build file to remove that Werror, it fails again later:

root@ubuntui386:~/mesa# PKG_CONFIG_PATH="/usr/local/mesa32/lib/pkgconfig" ninja -C build
ninja: Entering directory `build'
[4/4] Linking target src/gallium/targets/dri/libgallium_dri.so.
FAILED: src/gallium/targets/dri/libgallium_dri.so 
/usr/bin/g++-8  -o src/gallium/targets/dri/libgallium_dri.so 'src/gallium/targets/dri/gallium_dri@sha/target.c.o' -Wl,--no-undefined -Wl,--as-needed -shared -fPIC -Wl,--start-group -Wl,-soname,libgallium_dri.so -g -O2 -fstack-protector-strong -Wformat -Werror=format-security -Wall -Wl,-Bsymbolic-functions -Wl,-z,relro src/mesa/libmesa_gallium.a src/compiler/glsl/libglsl.a src/compiler/glsl/glcpp/libglcpp.a src/util/libmesa_util.a src/compiler/nir/libnir.a src/compiler/libcompiler.a src/mesa/libmesa_sse41.a src/mesa/drivers/dri/common/libdricommon.a src/mesa/drivers/dri/common/libmegadriver_stub.a src/gallium/state_trackers/dri/libdri.a src/gallium/auxiliary/libgalliumvl.a src/gallium/auxiliary/libgallium.a src/mapi/shared-glapi/libglapi.so.0.0.0 src/gallium/auxiliary/pipe-loader/libpipe_loader_static.a src/loader/libloader.a src/util/libxmlconfig.a src/gallium/winsys/sw/null/libws_null.a src/gallium/winsys/sw/wrapper/libwsw.a src/gallium/winsys/sw/dri/libswdri.a src/gallium/winsys/sw/kms-dri/libswkmsdri.a src/gallium/drivers/llvmpipe/libllvmpipe.a src/gallium/drivers/softpipe/libsoftpipe.a src/gallium/drivers/iris/libiris.a src/gallium/drivers/iris/liblibiris_gen80.a src/gallium/drivers/iris/liblibiris_gen90.a src/gallium/drivers/iris/liblibiris_gen100.a src/gallium/drivers/iris/liblibiris_gen110.a src/intel/common/libintel_common.a src/intel/isl/libisl.a src/intel/isl/libisl_gen40.a src/intel/isl/libisl_gen50.a src/intel/isl/libisl_gen60.a src/intel/isl/libisl_gen70.a src/intel/isl/libisl_gen75.a src/intel/isl/libisl_gen80.a src/intel/isl/libisl_gen90.a src/intel/isl/libisl_gen100.a src/intel/isl/libisl_gen110.a src/intel/isl/libisl_tiled_memcpy.a src/intel/isl/libisl_tiled_memcpy_sse41.a src/intel/compiler/libintel_compiler.a src/intel/dev/libintel_dev.a src/intel/blorp/libblorp.a src/gallium/winsys/iris/drm/libiriswinsys.a -pthread -Wl,--gc-sections -Wl,--version-script /root/mesa/src/gallium/targets/dri/dri.sym -Wl,--dynamic-list /root/mesa/src/gallium/targets/dri/../dri-vdpau.dyn -lexpat -ldrm -Wl,--end-group -L/usr/lib/llvm-7/lib -lLLVM-7 -lz -lm -ldrm -ldrm -ldrm -ldrm -L/usr/lib/llvm-7/lib -lLLVM-7 -lunwind -ldl -lm -lsensors -ldrm -ldrm -lexpat -lm -lz -lm -ldrm -L/usr/lib/llvm-7/lib -lLLVM-7 -ldrm -ldrm -ldrm -ldrm -ldrm -lexpat -ldrm '-Wl,-rpath,$ORIGIN/../../../mapi/shared-glapi' -Wl,-rpath-link,/root/mesa/build/src/mapi/shared-glapi  
src/gallium/drivers/iris/libiris.a(iris_bufmgr.c.o): In function `gen_clflush_range':
/root/mesa/build/../src/intel/common/gen_clflush.h:37: undefined reference to `__builtin_ia32_clflush'
src/gallium/drivers/iris/libiris.a(iris_bufmgr.c.o): In function `gen_invalidate_range':
/root/mesa/build/../src/intel/common/gen_clflush.h:64: undefined reference to `__builtin_ia32_clflush'
/root/mesa/build/../src/intel/common/gen_clflush.h:65: undefined reference to `__builtin_ia32_mfence'
collect2: error: ld returned 1 exit status
ninja: build stopped: subcommand failed.

Ken has a fix:

https://gitlab.freedesktop.org/kwg/mesa/commit/0d2c4e294ee935ce744b2a4a2ff2dad35c8d019e

Fixed by:


e2cb0c5e0e11b586a793a2b9d1b632d97fa1b6db
Author:     Kenneth Graunke <kenneth@whitecape.org>
AuthorDate: Thu Feb 21 14:29:00 2019 -0800
Commit:     Kenneth Graunke <kenneth@whitecape.org>
CommitDate: Thu Feb 21 14:51:15 2019 -0800

Parent:     7272fe9c086 intel/fs: Rely on undocumented unrestricted regioning for 32x16-bit integer multiply.
Merged:     djdeath/max-image-fix
Contained:  master
Follows:    19.0-branchpoint (1333)

iris: Enable -msse2 and -mstackrealign

This is needed for gen_clflush.h intrinsics to work on 32-bit builds.
i965 and anv both set these, and iris needs to as well.

Tested-by: Mark Janes <mark.a.janes@intel.com>