Started a cmake tutorial

Step4/MathFunctions/CMakeLists.txt

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+# create the MathFunctions library
+add_library(MathFunctions MathFunctions.cxx)
+
+# state that anybody linking to us needs to include the current source dir
+# to find MathFunctions.h, while we don't.
+target_include_directories(MathFunctions
+                           INTERFACE ${CMAKE_CURRENT_SOURCE_DIR}
+                           )
+
+# should we use our own math functions
+option(USE_MYMATH "Use tutorial provided math implementation" ON)
+if (USE_MYMATH)
+  target_compile_definitions(MathFunctions PRIVATE "USE_MYMATH")
+
+  # library that just does sqrt
+  add_library(SqrtLibrary STATIC
+              mysqrt.cxx
+              )
+
+  # link SqrtLibrary to tutorial_compiler_flags
+  target_link_libraries(SqrtLibrary PUBLIC tutorial_compiler_flags)
+
+  target_link_libraries(MathFunctions PRIVATE SqrtLibrary)
+endif()
+
+# link MathFunctions to tutorial_compiler_flags
+target_link_libraries(MathFunctions PUBLIC tutorial_compiler_flags)

Step4/MathFunctions/MathFunctions.cxx

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+#include "MathFunctions.h"
+
+#include <cmath>
+
+#ifdef USE_MYMATH
+#  include "mysqrt.h"
+#endif
+
+namespace mathfunctions {
+double sqrt(double x)
+{
+// which square root function should we use?
+#ifdef USE_MYMATH
+  return detail::mysqrt(x);
+#else
+  return std::sqrt(x);
+#endif
+}
+}

Step4/MathFunctions/MathFunctions.h

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+#pragma once
+
+namespace mathfunctions {
+double sqrt(double x);
+}

Step4/MathFunctions/mysqrt.cxx

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+#include "mysqrt.h"
+
+#include <iostream>
+
+namespace mathfunctions {
+namespace detail {
+// a hack square root calculation using simple operations
+double mysqrt(double x)
+{
+  if (x <= 0) {
+    return 0;
+  }
+
+  double result = x;
+
+  // do ten iterations
+  for (int i = 0; i < 10; ++i) {
+    if (result <= 0) {
+      result = 0.1;
+    }
+    double delta = x - (result * result);
+    result = result + 0.5 * delta / result;
+    std::cout << "Computing sqrt of " << x << " to be " << result << std::endl;
+  }
+  return result;
+}
+}
+}

Step4/MathFunctions/mysqrt.h

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+#pragma once
+
+namespace mathfunctions {
+namespace detail {
+double mysqrt(double x);
+}
+}