message: CMake config: check whether variable is DEFINED
  When running CMake with `-Werror=dev` on an external project that uses/finds
  LIEF, there is an error if the user did not define the `LIEF_INCLUDE_DIR`
  variable. This variable is not defined by default.
  This commit adds a check for whether the variable is defined.
diff: @@ -165,7 +165,8 @@ set(LIEF_LIBRARIES LIEF::LIEF)
  get_target_property(LIEF_INCLUDE_DIRS LIEF::LIEF INTERFACE_INCLUDE_DIRECTORIES)
  # Set the following for backwards compatibility
  -if(LIEF_INCLUDE_DIR AND EXISTS "${LIEF_INCLUDE_DIR}/LIEF/version.h")
  +if(DEFINED LIEF_INCLUDE_DIR)
  + if(EXISTS "${LIEF_INCLUDE_DIR}/LIEF/version.h")
  file(STRINGS "${LIEF_INCLUDE_DIR}/LIEF/version.h" LIEF_H REGEX "^#define LIEF_VERSION \"[^\"]*\"$")
  string(REGEX REPLACE "^.*LIEF_VERSION \"([0-9]+).*$" "\\1" LIEF_VERSION_MAJOR "${LIEF_H}")
  @@ -177,3 +178,4 @@ if(LIEF_INCLUDE_DIR AND EXISTS "${LIEF_INCLUDE_DIR}/LIEF/version.h")
  set(LIEF_MINOR_VERSION "${LIEF_VERSION_MINOR}")
  set(LIEF_PATCH_VERSION "${LIEF_VERSION_PATCH}")
  endif()
  +endif()
  