build: add exports targets for build tree usage

This enables the use of the export targets from the build tree to allow
easy use of the CMark library in other projects.

Resolves: commonmark#307

Author: compnerd

src/CMakeLists.txt

@@ -136,6 +136,10 @@ if(CMARK_SHARED OR CMARK_STATIC)
     )
 
   install(EXPORT cmark DESTINATION ${CMAKE_INSTALL_LIBDIR}/cmake)
+
+  configure_file(cmarkConfig.cmake.in
+    ${CMAKE_CURRENT_BINARY_DIR}/cmarkConfig.cmake)
+  export(TARGETS ${CMARK_INSTALL} FILE cmarkTargets.cmake)
 endif()
 
 # Feature tests

src/cmarkConfig.cmake.in

@@ -0,0 +1,7 @@
+set(HAVE_LIBCMARK_STATIC @CMARK_STATIC@)
+set(HAVE_LIBCMARK_SHARED @CMARK_SHARED@)
+
+if((HAVE_LIBCMARK_STATIC AND NOT TARGET libcmark_static) OR
+   (HAVE_LIBCMARK_SHARED AND NOT TARGET libcmark))
+  include(cmarkTargets.cmake)
+endif()