chore: update build script

- change project layout for better understanding.
- update build script for more close to standard cmake way.

CMake/FindSTB.cmake

@@ -0,0 +1,52 @@
+# - Find STB library
+# Find the STB headers
+#
+# This module defines the following variables:
+#   STB_FOUND        - True if STB was found
+#   STB_INCLUDE_DIRS - Location of the STB headers
+#
+# This module defines the following imported targets:
+#   STB::STB - Header-only interface library for STB
+
+# STB_ROOT must be specified by the user
+if (NOT DEFINED STB_ROOT)
+    set(STB_FOUND FALSE)
+else ()
+    # Look for STB_image.h in the specified STB_ROOT directory
+    find_path(STB_INCLUDE_DIR
+        NAMES STB_image.h
+        HINTS ${STB_ROOT}
+        NO_DEFAULT_PATH
+    )
+
+    # Check find status
+    if(STB_INCLUDE_DIR)
+        set(STB_FOUND TRUE)
+        set(STB_INCLUDE_DIRS ${STB_INCLUDE_DIR})
+    else()
+        set(STB_FOUND FALSE)
+    endif()
+
+    # Hide intermediate variables
+    mark_as_advanced(STB_INCLUDE_DIR)
+endif ()
+
+# Check find result
+if (STB_FOUND)
+    # Add library
+    message(STATUS "Found STB library")
+    # Add library
+    add_library(STB INTERFACE IMPORTED)
+    # Add alias to it
+    add_library(STB::STB ALIAS STB)
+    # Setup header files
+    set_target_properties(STB PROPERTIES
+    INTERFACE_INCLUDE_DIRECTORIES
+        "${STB_INCLUDE_DIRS}"
+    )
+else ()
+    # If it is required, show infomations.
+    if (std_FIND_REQUIRED)
+        message(FATAL_ERROR "Fail to find STB library.")
+    endif ()
+endif ()