Avoid installing options.h with the other headers

This install is conflicting with the manual one done after.
It causes spurious recompilations for users as the original file
is copied and then overwritten with each install.

Fixes #1769

PiperOrigin-RevId: 692919849
Change-Id: I2fb9c68f6e547212425d5b5ae7205bbf19b46bf4

CMakeLists.txt

@@ -214,11 +214,13 @@ if(ABSL_ENABLE_INSTALL)
     )
     )
   endif()  # absl_VERSION
   endif()  # absl_VERSION
 
 
+  # Install the headers except for "options.h" which is installed separately.
   install(DIRECTORY absl
   install(DIRECTORY absl
     DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}
     DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}
     FILES_MATCHING
     FILES_MATCHING
       PATTERN "*.inc"
       PATTERN "*.inc"
       PATTERN "*.h"
       PATTERN "*.h"
+      PATTERN "options.h" EXCLUDE
       PATTERN "copts" EXCLUDE
       PATTERN "copts" EXCLUDE
       PATTERN "testdata" EXCLUDE
       PATTERN "testdata" EXCLUDE
     )
     )
@@ -258,7 +260,21 @@ if(ABSL_ENABLE_INSTALL)
     ABSL_INTERNAL_OPTIONS_H_PINNED
     ABSL_INTERNAL_OPTIONS_H_PINNED
     "${ABSL_INTERNAL_OPTIONS_H_CONTENTS}")
     "${ABSL_INTERNAL_OPTIONS_H_CONTENTS}")
 
 
-  file(GENERATE OUTPUT "${CMAKE_BINARY_DIR}/options-pinned.h" CONTENT "${ABSL_INTERNAL_OPTIONS_H_PINNED}")
+  # If the file already exists, check if it matches the new contents.
+  # This avoids writing the file if it is already up-to-date when the CMake
+  # generation is triggered and triggering unnecessary rebuilds.
+  set(ABSL_INTERNAL_OPTIONS_H_PINNED_NEEDS_UPDATE TRUE)
+  if (EXISTS "${CMAKE_BINARY_DIR}/options-pinned.h")
+    file(READ "${CMAKE_BINARY_DIR}/options-pinned.h" ABSL_INTERNAL_OPTIONS_PINNED_H_CONTENTS)
+    if ("${ABSL_INTERNAL_OPTIONS_H_PINNED}" STREQUAL "${ABSL_INTERNAL_OPTIONS_PINNED_H_CONTENTS}")
+      set(ABSL_INTERNAL_OPTIONS_H_PINNED_NEEDS_UPDATE FALSE)
+    endif()
+  endif()
+
+  # If the file needs an update, generate it.
+  if (ABSL_INTERNAL_OPTIONS_H_PINNED_NEEDS_UPDATE)
+    file(GENERATE OUTPUT "${CMAKE_BINARY_DIR}/options-pinned.h" CONTENT "${ABSL_INTERNAL_OPTIONS_H_PINNED}")
+  endif()
 
 
   install(FILES "${CMAKE_BINARY_DIR}/options-pinned.h"
   install(FILES "${CMAKE_BINARY_DIR}/options-pinned.h"
          DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/absl/base
          DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/absl/base