Statements that begin with # are preprocessor directives. They are processed before the compiler sees the source code. For example, #include <afile> causes the contents of afile to be pasted directly into the source at the point of inclusion. #define myLength 10 causes every occurrence of the word, myLength, to be replaced with the text, 10. All these things are done before the compiler compiles, the linker links.
In the case of #if defined INTEL_W32 (or, usually, just #ifdef INTEL_W32), any material between the #if and a #else or #endif will be included in the source code IF there has been a previous statement defining INTEL_W32. If that value has not been defined, the material will not be included. Obviously, if the material is not included, the code will not be compiled, no executable statements will result.
You should take an hour or so and learn what actually happens in the steps that occur between the pounding of the keyboard and the execution of machine-code statements.