build: check if building .h only lib (interface) or static lib

CMakeLists.txt

@@ -43,10 +43,32 @@ if (SEALLIB_VFS)
     list(APPEND SEALLIB_SOURCES "src/lib/seallib/vfs.cpp" "src/lib/seallib/vfs.h")
 endif()
 
-add_library(seallib STATIC ${SEALLIB_SOURCES})
+set(SEALLIB_HAS_SOURCES FALSE)
+foreach(src ${SEALLIB_SOURCES})
+    if(src MATCHES "\\.cpp$")
+        set(SEALLIB_HAS_SOURCES TRUE)
+        break()
+    endif()
+endforeach()
+
+if (SEALLIB_HAS_SOURCES)
+    SEALLIB_HAS_SOURCES(seallib STATIC ${SEALLIB_SOURCES})
+else()
+    add_library(seallib INTERFACE)
+    target_sources(seallib INTERFACE ${SEALLIB_SOURCES})
+endif()
+
+if(SEALLIB_HAS_SOURCES)
+    set(SEALLIB_SCOPE PUBLIC)
+else()
+    set(SEALLIB_SCOPE INTERFACE)
+endif()
+
 set_target_properties(seallib PROPERTIES LINKER_LANGUAGE CXX)
-target_include_directories(seallib PUBLIC src/lib)
+
-target_compile_features(seallib PUBLIC cxx_std_20)
+target_include_directories(seallib ${SEALLIB_SCOPE} src/lib)
+target_compile_features(seallib ${SEALLIB_SCOPE} cxx_std_20)
+
 set_target_properties(seallib PROPERTIES CXX_EXTENSIONS OFF)
 
 # ------------------------------