PR #1715: Generate options.h using CMake only once

Imported from GitHub PR #1715

Using abseil via CMake ExternalProject, for each compilation abseil is reconfigured. This leads to a new options-pinned.h generated and then a new options.h installed. Because of that new options.h file, all projects depending on abseil recompile entirely.

Using file(GENERATE ...) instead of file(WRITE ...) allows generating options-pinned.h only when the content is new or changed.

Merge b3b5386 into cd75cb4

Merging this change closes #1715

COPYBARA_INTEGRATE_REVIEW=#1715 from BotellaA:patch-1 b3b5386
PiperOrigin-RevId: 652495570
Change-Id: Ib14bd9a78f80a540aae73d7a94cb46e2c05e8509

CMakeLists.txt

@@ -249,7 +249,7 @@ if(ABSL_ENABLE_INSTALL)
     ABSL_INTERNAL_OPTIONS_H_PINNED
     "${ABSL_INTERNAL_OPTIONS_H_CONTENTS}")
 
-  file(WRITE "${CMAKE_BINARY_DIR}/options-pinned.h" "${ABSL_INTERNAL_OPTIONS_H_PINNED}")
+  file(GENERATE OUTPUT "${CMAKE_BINARY_DIR}/options-pinned.h" CONTENT "${ABSL_INTERNAL_OPTIONS_H_PINNED}")
 
   install(FILES "${CMAKE_BINARY_DIR}/options-pinned.h"
          DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/absl/base