feat(cmake): add export support (#523)

* feat: add export support

This makes sdbus-c++ consumable from the build tree without the need for
an explicit installation. This can be very handy for e.g. testing
purposes. Crucially, downstream projects will depend on upstream to
properly export everything. Otherwise, they can't be built as static
libraries, even if sdbus-c++ is only a private dependency.

See also https://runebook.dev/en/docs/cmake/command/export

* fix(cmake): avoid possibly empty EXPORT_SET

EXPORT_SET is empty when SDBUSCPP_INSTALL is OFF,
which leads to targets file with no targets.

---------

Co-authored-by: Stanislav Angelovič <stanislav.angelovic@protonmail.com>

Author: Krzmbrzl

CMakeLists.txt

@@ -284,14 +284,23 @@ set(PKGCONFIG_DEPS ${SDBUS_LIB})
 configure_file(pkgconfig/sdbus-c++.pc.in pkgconfig/sdbus-c++.pc @ONLY)
 
 #----------------------------------
-# INSTALLATION
+# EXPORTING BUILD-TREE TARGETS
+#----------------------------------
+
+set(EXPORT_SET sdbus-c++)
+if(NOT BUILD_SHARED_LIBS)
+    list(APPEND EXPORT_SET "sdbus-c++-objlib")
+endif()
+
+export(TARGETS ${EXPORT_SET}
+       NAMESPACE SDBusCpp::
+       FILE sdbus-c++-targets.cmake)
+
+#----------------------------------
+# INSTALLATION & EXPORTING INSTALL-TREE TARGETS
 #----------------------------------
 
 if(SDBUSCPP_INSTALL)
-    set(EXPORT_SET sdbus-c++)
-    if(NOT BUILD_SHARED_LIBS)
-        list(APPEND EXPORT_SET "sdbus-c++-objlib")
-    endif()
     install(TARGETS ${EXPORT_SET}
             EXPORT sdbus-c++-targets
             RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR} COMPONENT sdbus-c++-runtime