autotools: CMake package version compatibility checking

Closes #523.

Copied CMake's bundled AnyNewerVersion template to our local cmake/ directory, and modified both CMake and autotools scripts to configure/install it.

.gitignore

@@ -57,6 +57,7 @@
 /c++/capnp
 /c++/capnp-evolution-test
 /c++/cmake/CapnProtoConfig.cmake
+/c++/cmake/CapnProtoConfigVersion.cmake
 /c++/pkgconfig/*.pc
 /c++/capnp-test
 /c++/capnpc-c++

c++/CMakeLists.txt

@@ -81,11 +81,17 @@ add_subdirectory(src)
 # Install ======================================================================
 
 include(CMakePackageConfigHelpers)
-write_basic_package_version_file(
-  "${CMAKE_CURRENT_BINARY_DIR}/cmake/CapnProtoConfigVersion.cmake"
-  VERSION ${VERSION}
-  COMPATIBILITY AnyNewerVersion
-)
+
+# We used to use write_basic_package_version_file(), but since the autotools build needs to install
+# a config version script as well, I copied the AnyNewerVersion template from my CMake Modules
+# directory to Cap'n Proto's cmake/ directory (alternatively, we could make the autotools build
+# depend on CMake).
+#
+# We might as well use the local copy of the template. In the future we can modify the project's
+# version compatibility policy just by changing that file.
+set(PACKAGE_VERSION ${VERSION})
+configure_file(cmake/CapnProtoConfigVersion.cmake.in cmake/CapnProtoConfigVersion.cmake @ONLY)
+
 set(CONFIG_PACKAGE_LOCATION ${CMAKE_INSTALL_LIBDIR}/cmake/CapnProto)
 
 configure_package_config_file(cmake/CapnProtoConfig.cmake.in

c++/cmake/CapnProtoConfigVersion.cmake.in

+# This is a copy of /usr/share/cmake-3.5/Modules/BasicConfigVersion-AnyNewerVersion.cmake.in, with
+# the following change:
+#   - @CVF_VERSION renamed to @PACKAGE_VERSION@. Autoconf defines a PACKAGE_VERSION
+#     output variable for us, so might as well take advantage of that.
+
+# This is a basic version file for the Config-mode of find_package().
+# It is used by write_basic_package_version_file() as input file for configure_file()
+# to create a version-file which can be installed along a config.cmake file.
+#
+# The created file sets PACKAGE_VERSION_EXACT if the current version string and
+# the requested version string are exactly the same and it sets
+# PACKAGE_VERSION_COMPATIBLE if the current version is >= requested version.
+# The variable PACKAGE_VERSION must be set before calling configure_file().
+
+set(PACKAGE_VERSION "@PACKAGE_VERSION@")
+
+if(PACKAGE_VERSION VERSION_LESS PACKAGE_FIND_VERSION)
+  set(PACKAGE_VERSION_COMPATIBLE FALSE)
+else()
+  set(PACKAGE_VERSION_COMPATIBLE TRUE)
+  if(PACKAGE_FIND_VERSION STREQUAL PACKAGE_VERSION)
+    set(PACKAGE_VERSION_EXACT TRUE)
+  endif()
+endif()
+
+# if the installed or the using project don't have CMAKE_SIZEOF_VOID_P set, ignore it:
+if("${CMAKE_SIZEOF_VOID_P}" STREQUAL "" OR "@CMAKE_SIZEOF_VOID_P@" STREQUAL "")
+   return()
+endif()
+
+# check that the installed version has the same 32/64bit-ness as the one which is currently searching:
+if(NOT CMAKE_SIZEOF_VOID_P STREQUAL "@CMAKE_SIZEOF_VOID_P@")
+   math(EXPR installedBits "@CMAKE_SIZEOF_VOID_P@ * 8")
+   set(PACKAGE_VERSION "${PACKAGE_VERSION} (${installedBits}bit)")
+   set(PACKAGE_VERSION_UNSUITABLE TRUE)
+endif()

c++/cmake/CapnProtoTargets.cmake

@@ -12,8 +12,6 @@
 #
 #   -DCapnProto_DEBUG=ON
 #
-# TODO(soon): Implement CapnProtoConfigVersion.cmake, too.
-#
 # TODO(someday): Distinguish between debug and release builds. I.e., set IMPORTED_LOCATION_RELEASE
 #   rather than IMPORTED_LOCATION, etc., if this installation was configured as a release build. But
 #   how do we tell? grep for -g in CXXFLAGS?

c++/configure.ac

@@ -116,6 +116,7 @@ AC_DEFUN([CAPNP_PKG_CONFIG_FILES], [ \
 ])
 AC_DEFUN([CAPNP_CMAKE_CONFIG_FILES], [ \
   cmake/CapnProtoConfig.cmake \
+  cmake/CapnProtoConfigVersion.cmake \
 ])
 
 [CAPNP_PKG_CONFIG_FILES]="CAPNP_PKG_CONFIG_FILES"
@@ -129,5 +130,10 @@ PACKAGE_CMAKE_INSTALL_FULL_INCLUDEDIR="${CMAKE_CURRENT_LIST_DIR}/../../include"
 AC_SUBST([PACKAGE_INIT])
 AC_SUBST([PACKAGE_CMAKE_INSTALL_FULL_INCLUDEDIR])
 
+# CapnProtoConfigVersion.cmake.in needs PACKAGE_VERSION (already defined by AC_INIT) and
+# CMAKE_SIZEOF_VOID_P output variables.
+AC_CHECK_SIZEOF([void *])
+AC_SUBST(CMAKE_SIZEOF_VOID_P, $ac_cv_sizeof_void_p)
+
 AC_CONFIG_FILES([Makefile] CAPNP_PKG_CONFIG_FILES CAPNP_CMAKE_CONFIG_FILES)
 AC_OUTPUT