organise the build setup nicely

meson.build

 project('hexboard-cpp', 'cpp')
 
-src = ['src/main.cpp', 'src/lib/State.cpp']
-executable('hexboard-cpp', src)
-
+subdir('src')
+subdir('test')
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src/main.cpp

@@ -5,7 +5,7 @@
  *      Author: webpigeon
  */
 
-#include <src/lib/state.h>
+#include "lib/state.h"
 #include <iostream>
 
 using namespace std;

src/meson.build

+src = ['main.cpp', 'lib/state.cpp']
+e = executable('hexboard-cpp', src)
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test/hex_test.cpp

+#define DOCTEST_CONFIG_IMPLEMENT_WITH_MAIN
+
+#include "doctest.h"

test/meson.build

+testexe = executable('hexboard-test', 'hex_test.cpp')
+test('hexboard', testexe)
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