Apply LTS transformations for 20260518 LTS branch (#2056)

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 3583e3d..e130bf5 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -23,8 +23,8 @@
   cmake_policy(SET CMP0141 NEW)
 endif (POLICY CMP0141)
 
-project(absl LANGUAGES CXX)
-set(ABSL_SOVERSION 0)
+project(absl LANGUAGES CXX VERSION 20260518)
+set(ABSL_SOVERSION "2605.0.0")
 include(CTest)
 
 # Output directory is correct by default for most build setups. However, when
@@ -170,17 +170,7 @@
 add_subdirectory(absl)
 
 if(ABSL_ENABLE_INSTALL)
-  # absl:lts-remove-begin(system installation is supported for LTS releases)
-  # We don't support system-wide installation
-  list(APPEND SYSTEM_INSTALL_DIRS "/usr/local" "/usr" "/opt/" "/opt/local" "c:/Program Files/${PROJECT_NAME}")
-  if(NOT DEFINED CMAKE_INSTALL_PREFIX OR CMAKE_INSTALL_PREFIX IN_LIST SYSTEM_INSTALL_DIRS)
-    message(WARNING "\
-  The default and system-level install directories are unsupported except in LTS \
-  releases of Abseil.  Please set CMAKE_INSTALL_PREFIX to install Abseil in your \
-  source or build tree directly.\
-    ")
-  endif()
-  # absl:lts-remove-end
+  
 
   # install as a subdirectory only
   install(EXPORT ${PROJECT_NAME}Targets

MODULE.bazel

diff --git a/MODULE.bazel b/MODULE.bazel
index cc60d22..6965002 100644
--- a/MODULE.bazel
+++ b/MODULE.bazel
@@ -16,7 +16,7 @@
 
 module(
     name = "abseil-cpp",
-    version = "head",
+    version = "20260518.0",
 )
 
 cc_configure = use_extension("@rules_cc//cc:extensions.bzl",

absl/base/config.h

diff --git a/absl/base/config.h b/absl/base/config.h
index d4a7bfb..a373164 100644
--- a/absl/base/config.h
+++ b/absl/base/config.h
@@ -117,8 +117,8 @@
 //
 // LTS releases can be obtained from
 // https://github.com/abseil/abseil-cpp/releases.
-#undef ABSL_LTS_RELEASE_VERSION
-#undef ABSL_LTS_RELEASE_PATCH_LEVEL
+#define ABSL_LTS_RELEASE_VERSION 20260518
+#define ABSL_LTS_RELEASE_PATCH_LEVEL 0
 
 // Helper macro to convert a CPP variable to a string literal.
 #define ABSL_INTERNAL_DO_TOKEN_STR(x) #x

absl/base/options.h

diff --git a/absl/base/options.h b/absl/base/options.h
index 033eac3..8f4de67 100644
--- a/absl/base/options.h
+++ b/absl/base/options.h
@@ -150,8 +150,8 @@
 // be changed to a new, unique identifier name.  In particular "head" is not
 // allowed.
 
-#define ABSL_OPTION_USE_INLINE_NAMESPACE 0
-#define ABSL_OPTION_INLINE_NAMESPACE_NAME head
+#define ABSL_OPTION_USE_INLINE_NAMESPACE 1
+#define ABSL_OPTION_INLINE_NAMESPACE_NAME lts_20260518
 
 // ABSL_OPTION_HARDENED
 //