Introduce the build interface param for cpp_libraray()

Author: Bill-hbrhbr

CMake/ystdlib-cpp-helpers.cmake

@@ -1,26 +1,34 @@
-# Set up include paths for building the project and for external projects that incorporate this
-# project using the add_subdirectory() function.
-set(CPP_LIB_BUILD_INTERFACE "${PROJECT_SOURCE_DIR}/src")
-
 # Adds a c++20 interface library in the subdirectory NAME with the target NAME and alias
 # NAMESPACE::NAME. Libraries with multiple levels of namespace nesting are currently not supported.
 #
 # @param NAME
 # @param NAMESPACE
+# @param [LIB_BUILD_INTERFACE] The list of include paths for building the library and for external
+#                              projects that link against it via the add_subdirectory() function.
 function(cpp_library)
     set(options "")
     set(oneValueArgs
         NAME
         NAMESPACE
     )
-    set(multiValueArgs "")
+    set(multiValueArgs LIB_BUILD_INTERFACE)
     cmake_parse_arguments(arg_cpp_lib "${options}" "${oneValueArgs}" "${multiValueArgs}" ${ARGN})
 
+    # TODO: Turn this into a function for handling other optional params that have default values.
+    if("LIB_BUILD_INTERFACE" IN_LIST arg_cpp_lib_KEYWORDS_MISSING_VALUES)
+        message(
+            FATAL_ERROR
+            "Missing build interface list for ${arg_cpp_lib_NAMESPACE}::${arg_cpp_lib_NAME}."
+        )
+    elseif(NOT DEFINED arg_cpp_lib_LIB_BUILD_INTERFACE)
+        set(arg_cpp_lib_LIB_BUILD_INTERFACE "${PROJECT_SOURCE_DIR}/src")
+    endif()
+
     add_library(${arg_cpp_lib_NAME} INTERFACE)
     target_include_directories(
         ${arg_cpp_lib_NAME}
         INTERFACE
-            "$<BUILD_INTERFACE:${CPP_LIB_BUILD_INTERFACE}>"
+            "$<BUILD_INTERFACE:${arg_cpp_lib_LIB_BUILD_INTERFACE}>"
     )
     target_compile_features(${arg_cpp_lib_NAME} INTERFACE cxx_std_20)
     add_library(${arg_cpp_lib_NAMESPACE}::${arg_cpp_lib_NAME} ALIAS ${arg_cpp_lib_NAME})