Add the RE2_BUILD_FRAMEWORK option to build RE2 as a framework.

See https://groups.google.com/g/re2-dev/c/P6_NM0YIWvA for details.
This has no effect unless RE2 is being built for an Apple platform
such as macOS or iOS.

Credit and gratitude to Yi Wang for proposing and explaining this.

Change-Id: I881e5707fd43cbfcef04e048ca68bcb6da9846ed
Reviewed-on: https://code-review.googlesource.com/c/re2/+/61031
Reviewed-by: Alex Chernyakhovsky <[email protected]>
Reviewed-by: Paul Wankadia <[email protected]>

CMakeLists.txt

@@ -17,6 +17,11 @@ option(RE2_USE_ICU "build against ICU for full Unicode properties support" OFF)
 # For historical reasons, this is just "USEPCRE", not "RE2_USE_PCRE".
 option(USEPCRE "build against PCRE for testing and benchmarking" OFF)
 
+# See https://groups.google.com/g/re2-dev/c/P6_NM0YIWvA for details.
+# This has no effect unless RE2 is being built for an Apple platform
+# such as macOS or iOS.
+option(RE2_BUILD_FRAMEWORK "build RE2 as a framework" OFF)
+
 # CMake seems to have no way to enable/disable testing per subproject,
 # so we provide an option similar to BUILD_TESTING, but just for RE2.
 option(RE2_BUILD_TESTING "enable testing for RE2" ON)
@@ -109,6 +114,13 @@ set_target_properties(re2 PROPERTIES PUBLIC_HEADER "${RE2_HEADERS}")
 set_target_properties(re2 PROPERTIES SOVERSION ${SONAME} VERSION ${SONAME}.0.0)
 add_library(re2::re2 ALIAS re2)
 
+if(APPLE AND RE2_BUILD_FRAMEWORK)
+  set_target_properties(re2 PROPERTIES
+                        FRAMEWORK TRUE
+                        FRAMEWORK_VERSION A
+                        MACOSX_FRAMEWORK_IDENTIFIER com.googlesource.code.re2)
+endif()
+
 if(UNIX)
   target_link_libraries(re2 PUBLIC Threads::Threads)
 endif()
@@ -180,6 +192,7 @@ install(TARGETS re2
         ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR}
         LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}
         RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR}
+        FRAMEWORK DESTINATION ${CMAKE_INSTALL_LIBDIR}
         PUBLIC_HEADER DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/re2
         INCLUDES DESTINATION ${CMAKE_INSTALL_INCLUDEDIR})
 install(EXPORT re2Targets