Code cleanup: #include <> misuse.

[mholtrop]

See: https://gcc.gnu.org/onlinedocs/gcc-7.5.0/cpp/Include-Syntax.html

#include
This variant is used for system header files. It searches for a file named file in a standard list of system directories. You can prepend directories to this list with the -I option (see Invocation).

#include "file"
This variant is used for header files of your own program. It searches for a file named file first in the directory containing the current file, then in the quote directories and then the same directories used for . You can prepend directories to the list of quote directories with the -iquote option.

Can we make #include statements consistent, where <> is for system headers and "" is for user defined headers?

Some compilers are pickier than others (as in clang) and will not search #include <> files in local directories. Besides compiler pickiness, it is easer on the reader.