outputC.py: construct_Makefile_from_outC_function_dict - compilation failed when using include_dirs argument

[amnonjw]

I'm working on a windows PC, I tested the various NRPy+ examples successfully so far.

When adding directory path to the include_dirs argument, compilation errors occur.
Among the errors was that the gcc command suddely was not found.

I resolved the errors with the following correction.

• line 698, change the command by adding \" before and after the include_dir object: include_dirs_str += "-I\"" + include_dir + "\" "
• line 643, change the command to if "gcc" in CC, and now I call the routine with argument CC=<full path of the gcc file>

Additionally, I found that in the cmdline_helper.py file - the method "new_C_compile" fails because it needs to validate that an exe file was generated. For that I changed lines 137 and 147 to use the os.path.isfile method with exec_name+".exe"