Do not build containers with -mach=native/-mtune=native

In some cases, the machine building the containers will have
instruction sets that are not supported by the machines running the
containers, causing SIGILL.

CMakeLists.txt

@@ -260,8 +260,11 @@ else ()
 	message(STATUS "Valgrind headers not found -- disabling valgrind support")
 endif()
 
-enable_c_flag_if_avail(-mtune=native C_FLAGS_REL LWAN_HAVE_MTUNE_NATIVE)
-enable_c_flag_if_avail(-march=native C_FLAGS_REL LWAN_HAVE_MARCH_NATIVE)
+option(MTUNE_NATIVE "Build with -mtune=native/-march=native" "ON")
+if (MTUNE_NATIVE)
+    enable_c_flag_if_avail(-mtune=native C_FLAGS_REL LWAN_HAVE_MTUNE_NATIVE)
+    enable_c_flag_if_avail(-march=native C_FLAGS_REL LWAN_HAVE_MARCH_NATIVE)
+endif ()
 
 enable_c_flag_if_avail(-fstack-protector-explicit CMAKE_C_FLAGS
 	LWAN_HAVE_STACK_PROTECTOR_EXPLICIT)

Containerfile

@@ -3,7 +3,7 @@ RUN apk add --no-cache gcc make musl-dev cmake pkgconfig linux-headers \
       luajit-dev sqlite-dev zlib-dev brotli-dev zstd-dev
 COPY . /lwan
 WORKDIR /lwan/build
-RUN cmake .. -DCMAKE_BUILD_TYPE=Release
+RUN cmake .. -DCMAKE_BUILD_TYPE=Release -DMTUNE_NATIVE=OFF
 RUN make -j
 
 FROM docker.io/library/alpine:3.14.2

README.md

@@ -159,6 +159,10 @@ names provided in the "Optional dependencies"  section.
 The `-DUSE_SYSLOG=ON` option can be passed to CMake to also log to the system log
 in addition to the standard output.
 
+If you're building Lwan for a distribution, it might be wise to use the
+`-DMTUNE_NATIVE=OFF` option, otherwise the generated binary may fail to
+run on some computers.
+
 ### Tests
 
     ~/lwan/build$ make testsuite