feat(cmake): optional ENABLE_LTO (IPO) for Release-style builds

- New cmake/ETLLTO.cmake: CheckIPOSupported, gate on ENABLE_LTO, forbid
  combination with ENABLE_ASAN.
- ENABLE_LTO option declared after LEG_BUNDLED_LIB so bundled libs stay
  default-on when configuring with -DENABLE_LTO only.
- README: document -DENABLE_LTO=ON for packagers.

Co-authored-by: Tim Fox <timfox@outlook.com>

Author: cursoragent

CMakeLists.txt

@@ -136,6 +136,8 @@ LEG_BUNDLED_LIB(FREETYPE Freetype2 "BUILD_CLIENT AND FEATURE_FREETYPE")
 LEG_BUNDLED_LIB(SQLITE3 SQlite3 "BUILD_CLIENT OR BUILD_MOD")
 LEG_BUNDLED_LIB(CJSON cJSON "BUILD_ENGINE OR BUILD_MOD")
 
+option(ENABLE_LTO				"Enable IPO/LTO for Release-style builds (toolchain must support it)"	OFF)
+
 #-----------------------------------------------------------------
 # Setup
 #-----------------------------------------------------------------
@@ -174,6 +176,9 @@ include(cmake/ETLTargets.cmake)
 # Platform specific compiler settings
 include(cmake/ETLPlatform.cmake)
 
+# Optional LTO (after compiler flags are established)
+include(cmake/ETLLTO.cmake)
+
 # Source globs
 include(cmake/ETLSources.cmake)
 

README.md

@@ -151,6 +151,16 @@ make
 sudo make install   # optional; set install vars first
 ```
 
+**Release packaging — optional LTO**
+
+For **Release** / **RelWithDebInfo** / **MinSizeRel**, you can enable **link-time optimization** if your toolchain supports it (CMake IPO):
+
+```sh
+cmake -DENABLE_LTO=ON ..
+```
+
+Leave it **off** for normal development (faster links). It cannot be combined with **`ENABLE_ASAN`**.
+
 **Notes**
 
 - 32-bit builds may need **multilib / `-devel`** packages.

cmake/ETLLTO.cmake

@@ -0,0 +1,32 @@
+#-----------------------------------------------------------------
+# Optional link-time optimization (IPO / LTO)
+#-----------------------------------------------------------------
+#
+# Off by default: LTO increases link time and needs a capable toolchain.
+# When enabled, applies to Release-style configurations only.
+
+if(NOT ENABLE_LTO)
+	return()
+endif()
+
+if(ENABLE_ASAN)
+	message(FATAL_ERROR "ENABLE_LTO cannot be used together with ENABLE_ASAN.")
+endif()
+
+if(CMAKE_VERSION VERSION_LESS "3.9")
+	message(FATAL_ERROR "ENABLE_LTO requires CMake 3.9 or newer (CheckIPOSupported).")
+endif()
+
+include(CheckIPOSupported)
+check_ipo_supported(RESULT _etl_ipo_supported OUTPUT _etl_ipo_error)
+
+if(NOT _etl_ipo_supported)
+	message(WARNING "ENABLE_LTO=ON but IPO is not supported by this toolchain: ${_etl_ipo_error}")
+	return()
+endif()
+
+set(CMAKE_INTERPROCEDURAL_OPTIMIZATION_RELEASE ON)
+set(CMAKE_INTERPROCEDURAL_OPTIMIZATION_RELWITHDEBINFO ON)
+set(CMAKE_INTERPROCEDURAL_OPTIMIZATION_MINSIZEREL ON)
+
+message(STATUS "Link-time optimization (IPO/LTO) enabled for Release, RelWithDebInfo, MinSizeRel")