fix: portable build of native extensions to avoid SIGILL

- replaced -march=native with -march=x86-64 -mtune=generic by default in extconf.rb and terrain_downsample_extconf.rb
- added TPC_EXT_NATIVE=1 env to enable -march=native -mtune=native
- added TPC_EXT_LTO=1 env to enable -flto
- updated lerc_extension docs (en/ru) in Optimizations section

Author: Efreh

src/docs/en/lerc_extension.md

@@ -145,7 +145,7 @@ The extension is integrated into the main tile caching service (`config.ru`) and
 ### Optimizations
 - Memory pre-allocation (`reserve()`)
 - Direct pointer arithmetic in critical loops
-- Aggressive compiler flags (`-O3`, `-march=native`, `-flto`)
+- Compiler flags (`-O3`; optional arch/LTO flags depending on build)
 - RTTI disabled for size reduction
 
 ### Limitations

src/docs/ru/lerc_extension.md

@@ -145,7 +145,7 @@ LercFFI.lerc_to_mapbox_png(lerc_data) # => png_data
 ### Оптимизации
 - Предварительная аллокация памяти (`reserve()`)
 - Прямая арифметика указателей в критических циклах
-- Агрессивные флаги компилятора (`-O3`, `-march=native`, `-flto`)
+- Флаги компилятора (`-O3`; опциональные arch/LTO зависят от сборки)
 - Отключение RTTI для уменьшения размера
 
 ### Ограничения

src/ext/extconf.rb

@@ -1,6 +1,16 @@
 require "mkmf"
 
-$CXXFLAGS += " -std=c++23 -O3 -march=native -mtune=native -flto -Wno-unused-function"
+native = ENV["TPC_EXT_NATIVE"] == "1"
+lto    = ENV["TPC_EXT_LTO"] == "1"
+
+arch_flags =
+  if native
+    " -march=native -mtune=native"
+  else
+    " -march=x86-64 -mtune=generic"
+  end
+
+$CXXFLAGS += " -std=c++23 -O3#{arch_flags}#{lto ? ' -flto' : ''} -Wno-unused-function"
 $CXXFLAGS += " -Wall -Wextra -Wpedantic"
 $CXXFLAGS += " -fno-rtti"
 $CPPFLAGS += " -I/usr/local/include"

src/ext/terrain_downsample_extconf.rb

@@ -1,6 +1,16 @@
 require "mkmf"
 
-$CXXFLAGS += " -std=c++23 -O3 -march=native -mtune=native -flto -Wno-unused-function"
+native = ENV["TPC_EXT_NATIVE"] == "1"
+lto    = ENV["TPC_EXT_LTO"] == "1"
+
+arch_flags =
+  if native
+    " -march=native -mtune=native"
+  else
+    " -march=x86-64 -mtune=generic"
+  end
+
+$CXXFLAGS += " -std=c++23 -O3#{arch_flags}#{lto ? ' -flto' : ''} -Wno-unused-function"
 $CXXFLAGS += " -Wall -Wextra -Wpedantic"
 $CXXFLAGS += " -fno-rtti"
 $CPPFLAGS += " -I/usr/local/include"