What causes g++ ignoring nonexisting directory - c++

Question

I am learning to use make. I am trying to compile a program using g++ on my home machine that I have successfully compiled using xl on a school machine. I am trying to link with an external library that I built and placed in a separate directory. I am trying to include the header file, but g++ claims the include directory does not exist. The include directory definitely exists, tab complete fills it in and find locates it as well.
Why does g++ decide the directory does not exist? In general what causes a directory to be ignored? I want to know why this is breaking so I can better write makefiles. More importantly to me, how do I fix this and have g++ search for arbitrary header files in whatever directory I tell it to look in?
I have seen questions about g++ ignoring directories, but those were in the case of ignoring various system default locations. They usually solved their problem by removing a conflicting version or just re installing their tool. My problem is different because g++ is ignoring a user defined directory that actually exists.
src/a3.cc:1:17: fatal error: apf.h: No such file or directory
#include "apf.h"
$ find /home/USER/Documents/GitHub/core/apf/
/home/USER/Documents/GitHub/core/apf/
/home/USER/Documents/GitHub/core/apf/apfUserData.cc
....
/home/USER/Documents/GitHub/core/apf/apf.h
....
I have tried manually specifying the directory in the make file using -I
apf = /home/USER/Documents/Github/core/apf/
CFLAGS = $(CFLAGS) -Wall -g -I $(apf) --pedantic-errors --verbose
I have also tried setting CPLUS_INCLUDE_PATH
export CPLUS_INCLUDE_PATH = $(apf)
For each of these attempts I see the same error, that g++ is "ignoring nonexistant directory"
COLLECT_GCC=/usr/bin/g++
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
ignoring nonexistent directory "/home/USER/Documents/Github/core/apf/"
ignoring nonexistent directory "/usr/lib/gcc/x86_64-redhat-linux/4.9.2/include-fixed"
ignoring nonexistent directory "/usr/lib/gcc/x86_64-redhat-linux/4.9.2/../../../../x86_64-redhat-linux/include"
ignoring duplicate directory "/usr/local/include"
as it is a non-system directory that duplicates a system directory
#include "..." search starts here:
#include <...> search starts here:
./inc
/usr/lib/gcc/x86_64-redhat-linux/4.9.2/../../../../include/c++/4.9.2
/usr/lib/gcc/x86_64-redhat-linux/4.9.2/../../../../include/c++/4.9.2/x86_64-redhat-linux
/usr/lib/gcc/x86_64-redhat-linux/4.9.2/../../../../include/c++/4.9.2/backward
/usr/lib/gcc/x86_64-redhat-linux/4.9.2/include
/usr/local/include
/usr/include
End of search list.
GNU C++ (GCC) version 4.9.2 20150212 (Red Hat 4.9.2-6) (x86_64-redhat-linux)
compiled by GNU C version 4.9.2 20150212 (Red Hat 4.9.2-6), GMP version 6.0.0, MPFR version 3.1.2, MPC version 1.0.2
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 954ba53e83b294b646fa1b6b73a69682

Answer 1

It appears that you have a case sensitive file system. You have GitHub in some places and Github in others. Make them all match correctly and you'll be in better shape.