Make libigl targets installable only for toplevel projects by default (follow-up on #839).

CMakeLists.txt

@@ -12,6 +12,7 @@ endif()
 option(LIBIGL_BUILD_TESTS      "Build libigl unit test"        ${LIBIGL_TOPLEVEL_PROJECT})
 option(LIBIGL_BUILD_TUTORIALS  "Build libigl tutorial"         ${LIBIGL_TOPLEVEL_PROJECT})
 option(LIBIGL_BUILD_PYTHON     "Build libigl python bindings"  ${LIBIGL_TOPLEVEL_PROJECT})
+option(LIBIGL_EXPORT_TARGETS   "Export libigl CMake targets"   ${LIBIGL_TOPLEVEL_PROJECT})
 
 # USE_STATIC_LIBRARY speeds up the generation of multiple binaries,
 # at the cost of a longer initial compilation time
@@ -32,8 +33,8 @@ option(LIBIGL_WITH_XML               "Use XML"                      ON)
 option(LIBIGL_WITH_PYTHON            "Use Python"                   ${LIBIGL_BUILD_PYTHON})
 ### End
 
-set(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR})		
-set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR})		
+set(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR})
+set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR})
 set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR})
 
 ### conditionally compile certain modules depending on libraries found on the system

cmake/libigl.cmake

@@ -35,6 +35,7 @@ option(LIBIGL_WITH_TRIANGLE          "Use Triangle"                 OFF)
 option(LIBIGL_WITH_XML               "Use XML"                      OFF)
 option(LIBIGL_WITH_PYTHON            "Use Python"                   OFF)
 option(LIBIGL_WITHOUT_COPYLEFT       "Disable Copyleft libraries"   OFF)
+option(LIBIGL_EXPORT_TARGETS         "Export libigl CMake targets"  OFF)
 
 ################################################################################
 
@@ -429,6 +430,10 @@ endif()
 ################################################################################
 ### Install and export all modules
 
+if(NOT LIBIGL_EXPORT_TARGETS)
+  return()
+endif()
+
 function(install_dir_files dir_name)
   if (dir_name STREQUAL "core")
     set(subpath "")