Don't add include path for config.h to header-only libs

This causes CMake errors and CMake is acutally right, it makes
no sense because the config header is only available during build
but a header-only lib is not being built.

cmake/modules/ConfigHeader.cmake

@@ -55,8 +55,10 @@ configure_file(
 )
 
 # ensure generated include files can be included via #include "resources/config.h"
-foreach(TARGET_NAME ${TARGET_PREFIX}${META_PROJECT_NAME}${TARGET_SUFFIX} ${TARGET_PREFIX}${META_PROJECT_NAME}${TARGET_SUFFIX}_static ${TARGET_PREFIX}${META_PROJECT_NAME}${TARGET_SUFFIX}_tests ${TARGET_PREFIX}${META_PROJECT_NAME}${TARGET_SUFFIX}_testlib)
-    if(TARGET ${TARGET_NAME})
-        target_include_directories(${TARGET_NAME} PRIVATE "${CMAKE_CURRENT_BINARY_DIR}")
-    endif()
-endforeach()
+if(NOT META_HEADER_ONLY_LIB)
+    foreach(TARGET_NAME ${TARGET_PREFIX}${META_PROJECT_NAME}${TARGET_SUFFIX} ${TARGET_PREFIX}${META_PROJECT_NAME}${TARGET_SUFFIX}_static ${TARGET_PREFIX}${META_PROJECT_NAME}${TARGET_SUFFIX}_tests ${TARGET_PREFIX}${META_PROJECT_NAME}${TARGET_SUFFIX}_testlib)
+        if(TARGET ${TARGET_NAME})
+            target_include_directories(${TARGET_NAME} PRIVATE "${CMAKE_CURRENT_BINARY_DIR}")
+        endif()
+    endforeach()
+endif()