fix_third_party: set install libdir and fix spdlog includes

With the update of the third-party libraries, qcor's compilation
crashes when try to link to the gtest and spdlog installed by
xacc. This PR fix that and allows for compilation of qcor without
touching qcor's configuration files.

CMakeLists.txt

@@ -60,6 +60,7 @@ set(CMAKE_SKIP_INSTALL_RPATH OFF)
 set(CMAKE_SKIP_RPATH OFF)
 set(CMAKE_INSTALL_RPATH_USE_LINK_PATH ON)
 set(CMAKE_EXPORT_COMPILE_COMMANDS TRUE)
+set(CMAKE_INSTALL_LIBDIR "${CMAKE_INSTALL_PREFIX}/lib")
 
 LIST(FIND CMAKE_PLATFORM_IMPLICIT_LINK_DIRECTORIES "${CMAKE_INSTALL_PREFIX}/lib" isSystemDir)
 
@@ -160,7 +161,7 @@ find_package(LAPACK)
 # Dependencies:
 option(XACC_DEPS_EXTERNAL "Try find external dependencies (system installation) rather than tpls" OFF)
 
-set(SPDLOG_INCLUDE_DIR ${PROJECT_SOURCE_DIR}/tpls/spdlog/include)
+set(SPDLOG_INCLUDE_DIR ${PROJECT_SOURCE_DIR}/tpls/spdlog/include/)
 if (XACC_DEPS_EXTERNAL) 
   find_package(spdlog) 
   if (spdlog_FOUND) 
@@ -248,7 +249,7 @@ install(FILES "${CMAKE_SOURCE_DIR}/tpls/mpark-variant/variant.hpp" DESTINATION i
 install(FILES "${CMAKE_SOURCE_DIR}/tpls/taocpp/operators.hpp" DESTINATION include/xacc/)
 install(FILES "${NLOHMANN_INCLUDE_DIR}/nlohmann/json.hpp" DESTINATION include/xacc/)
 
-install(DIRECTORY "${SPDLOG_INCLUDE_DIR}" DESTINATION include)
+install(DIRECTORY "${SPDLOG_INCLUDE_DIR}" DESTINATION include/spdlog/)
 install(DIRECTORY "${CMAKE_SOURCE_DIR}/tpls/exprtk" DESTINATION include)
 install(DIRECTORY "${EIGEN_INCLUDE_DIR}" DESTINATION include)
 install(DIRECTORY "${CMAKE_SOURCE_DIR}/tpls/rapidjson" DESTINATION include)
@@ -318,4 +319,4 @@ if(XACC_CPACK_RPM_PLATFORM)
   endif()
 
   include(CPack)
-endif()
+endif()