Allow yaml-cpp to be installed from another project (#1007)

Changes YAML_CPP_INSTALL from a cmake_dependent_option to an option.

Fixes #756, #847, and #1011.

CMakeLists.txt

@@ -9,6 +9,11 @@ endif()
 
 project(YAML_CPP VERSION 0.7.0 LANGUAGES CXX)
 
+set(YAML_CPP_MAIN_PROJECT OFF)
+if(CMAKE_SOURCE_DIR STREQUAL PROJECT_SOURCE_DIR)
+  set(YAML_CPP_MAIN_PROJECT ON)
+endif()
+
 include(CMakePackageConfigHelpers)
 include(CMakeDependentOption)
 include(CheckCXXCompilerFlag)
@@ -17,6 +22,7 @@ include(GNUInstallDirs)
 option(YAML_CPP_BUILD_CONTRIB "Enable yaml-cpp contrib in library" ON)
 option(YAML_CPP_BUILD_TOOLS "Enable parse tools" ON)
 option(YAML_BUILD_SHARED_LIBS "Build yaml-cpp shared library" ${BUILD_SHARED_LIBS})
+option(YAML_CPP_INSTALL "Enable generation of yaml-cpp install targets" ${YAML_CPP_MAIN_PROJECT})
 option(YAML_CPP_FORMAT_SOURCE "Format source" ON)
 
 if (YAML_CPP_BUILD_TESTS)
@@ -30,9 +36,6 @@ endif()
 cmake_dependent_option(YAML_CPP_BUILD_TESTS
   "Enable yaml-cpp tests" ON
   "BUILD_TESTING;CMAKE_SOURCE_DIR STREQUAL PROJECT_SOURCE_DIR" OFF)
-cmake_dependent_option(YAML_CPP_INSTALL
-  "Enable generation of yaml-cpp install targets" ON
-  "CMAKE_SOURCE_DIR STREQUAL PROJECT_SOURCE_DIR" OFF)
 cmake_dependent_option(YAML_MSVC_SHARED_RT
   "MSVC: Build yaml-cpp with shared runtime libs (/MD)" ON
   "CMAKE_SYSTEM_NAME MATCHES Windows" OFF)