Allow saying find_package(XACC 1.0.0)

This creates a version file so that you can specify a version for
find_package(XACC), like find_package(XACC 1.0.0). For more info, see:
https://cmake.org/cmake/help/latest/command/find_package.html#config-mode-version-selection

CMakeLists.txt

@@ -244,7 +244,9 @@ set(PATCH_VERSION 0)
 # their CMake builds, and pass -DXACC_DIR=/path/to/install/xacc
 # and CMake loads include paths, libs, etc
 configure_file("${CMAKE_SOURCE_DIR}/cmake/xacc-config.cmake.in" "${CMAKE_BINARY_DIR}/xacc-config.cmake" @ONLY)
+configure_file("${CMAKE_SOURCE_DIR}/cmake/xacc-config-version.cmake.in" "${CMAKE_BINARY_DIR}/xacc-config-version.cmake" @ONLY)
 install(FILES "${CMAKE_BINARY_DIR}/xacc-config.cmake" DESTINATION .)
+install(FILES "${CMAKE_BINARY_DIR}/xacc-config-version.cmake" DESTINATION .)
 install(FILES "${CMAKE_SOURCE_DIR}/cmake/Modules/format.cmake" DESTINATION share/xacc/)
 install(FILES "${CMAKE_SOURCE_DIR}/tpls/mpark-variant/variant.hpp" DESTINATION include/xacc/)
 install(FILES "${CMAKE_SOURCE_DIR}/tpls/taocpp/operators.hpp" DESTINATION include/xacc/)

cmake/xacc-config-version.cmake.in

@@ -0,0 +1,8 @@
+set(PACKAGE_VERSION "@MAJOR_VERSION@.@MINOR_VERSION@.@PATCH_VERSION@")
+# The quotes for the right operand are intentional here: if CMake executes this
+# file without ${PACKAGE_FIND_VERSION} set, then if() will still get three
+# operands. Without the quotes, if() will only get two operands in that case,
+# throwing an error.
+if(${PACKAGE_VERSION} VERSION_GREATER_EQUAL "${PACKAGE_FIND_VERSION}")
+    set(PACKAGE_VERSION_COMPATIBLE 1)
+endif()

cmake/xacc-config.cmake.in

@@ -20,11 +20,7 @@ if(NOT TARGET format)
 endif()
 
 # Set version info
-set(XACC_VERSION_MAJOR @MAJOR_VERSION@)
-set(XACC_VERSION_MINOR @MINOR_VERSION@)
-set(XACC_VERSION_PATCH @PATCH_VERSION@)
 set(XACC_VERSION_SUFFIX @XACC_BUILD_VERSION@)
-set(XACC_VERSION "@MAJOR_VERSION@.@MINOR_VERSION@.@PATCH_VERSION@-@XACC_BUILD_VERSION@")
 
 set(XACC_LIBRARY_DIR "${XACC_ROOT}/lib")