What should I do to solve the GNAT GPS "library not found" problem and build my first "Hello World" program on my Mac?

I downloaded the GNAT Community 2019 and have installed on my Mac in my home folder "/Users/leon/opt/GNAT"

I run the command "gps" in the directory "/Users/leon/opt/GNAT/2019/bin". And GPS showed up, then I created a project, typed the "Hello World" code. Just as the Wikibook shows.

with Ada.Text_IO;

procedure Hello is
begin
   Ada.Text_IO.Put_Line("Hello, world!");
end Hello;

When I clicked the build button, the program just failed to build.

The following is the building output.

gprbuild -d -P/Users/leon/Documents/ada/helloworld.gpr /Users/leon/Documents/ada/src/hello.adb
Compile
   [Ada]          hello.adb
Bind
   [gprbind]      hello.bexch
   [Ada]          hello.ali
Link
   [link]         hello.adb
ld: library not found for -lSystem
collect2: error: ld returned 1 exit status
gprbuild: link of hello.adb failed
gprbuild: failed command was: /users/leon/opt/gnat/2019/bin/gcc hello.o b__hello.o -L/Users/leon/Documents/ada/obj/ -L/Users/leon/Documents/ada/obj/ -L/users/leon/opt/gnat/2019/lib/gcc/x86_64-apple-darwin17.7.0/8.3.1/adalib/ /users/leon/opt/gnat/2019/lib/gcc/x86_64-apple-darwin17.7.0/8.3.1/adalib/libgnat.a -Wl,-rpath,@executable_path/ -Wl,-rpath,@executable_path/../../..//opt/gnat/2019/lib/gcc/x86_64-apple-darwin17.7.0/8.3.1/adalib -o hello
[2020-02-28 22:36:48] process exited with status 4, elapsed time: 00.79s

Please help. I can't figure out what is wrong with the configuration or something. Thank you in advance.

guys. I found the solution myself.

On the program's download's page, above the program's link, there is a README file.

I ignored it at first. Just then I revisited the official site and read the README file and got the solution as following :

== Mac OS: Xcode is now needed ==

On Mac OS, GNAT Community 2019 requires Xcode version 10 or above to be
installed. Once you do have Xcode installed, if you still observe an error
of the form:

  ld: library not found for -lSystem

then you might have to execute the following:

  xcode-select -s /Applications/Xcode.app/Contents/Developer

I think this has really given me a great lesson. READ THE INSTALLATION MANUAL FIRST.

Thank you guys.