Add support for plain-C precompiled headers for vcxproj generator

#include <stdio.h>

int main()
{
  FILE* f = fopen("somefile","r");
  return 0;
}

TEMPLATE  = app
CONFIG   += console precompile_header_c
PRECOMPILED_HEADER  = stable.h
HEADERS   = stable.h
SOURCES   = main.c