FindBoost: Define Boost::boost for all boost versions

Modules/FindBoost.cmake

@@ -1573,7 +1573,7 @@ endif()
 #  Add imported targets
 # ------------------------------------------------------------------------
 
-if(Boost_FOUND AND _Boost_IMPORTED_TARGETS)
+if(Boost_FOUND)
   # For header-only libraries
   if(NOT TARGET Boost::boost)
     add_library(Boost::boost INTERFACE IMPORTED)
@@ -1584,7 +1584,7 @@ if(Boost_FOUND AND _Boost_IMPORTED_TARGETS)
   endif()
 
   foreach(COMPONENT ${Boost_FIND_COMPONENTS})
-    if(NOT TARGET Boost::${COMPONENT})
+    if(_Boost_IMPORTED_TARGETS AND NOT TARGET Boost::${COMPONENT})
       string(TOUPPER ${COMPONENT} UPPERCOMPONENT)
       if(Boost_${UPPERCOMPONENT}_FOUND)
         if(Boost_USE_STATIC_LIBS)

Tests/FindBoost/CMakeLists.txt

@@ -8,3 +8,14 @@ add_test(NAME FindBoost.Test COMMAND
   --build-options ${build_options}
   --test-command ${CMAKE_CTEST_COMMAND} -V -C $<CONFIGURATION>
   )
+
+add_test(NAME FindBoost.TestHeaders COMMAND
+  ${CMAKE_CTEST_COMMAND} -C $<CONFIGURATION>
+  --build-and-test
+  "${CMake_SOURCE_DIR}/Tests/FindBoost/TestHeaders"
+  "${CMake_BINARY_DIR}/Tests/FindBoost/TestHeaders"
+  ${build_generator_args}
+  --build-project TestFindBoostHeaders
+  --build-options ${build_options}
+  --test-command ${CMAKE_CTEST_COMMAND} -V -C $<CONFIGURATION>
+  )

Tests/FindBoost/TestHeaders/CMakeLists.txt

@@ -0,0 +1,10 @@
+cmake_minimum_required(VERSION 3.1)
+project(TestFindBoostHeaders CXX)
+include(CTest)
+
+find_package(Boost REQUIRED)
+
+add_executable(test_boost_headers_tgt main.cxx)
+target_link_libraries(test_boost_headers_tgt
+                      Boost::boost)
+add_test(NAME test_boost_headers_tgt COMMAND test_boost_headers_tgt)

Tests/FindBoost/TestHeaders/main.cxx

@@ -0,0 +1,9 @@
+#include <boost/any.hpp>
+
+int main() {
+  boost::any a;
+  a = 5;
+  a = std::string("A string");
+
+  return 0;
+}