I was successfully able to get ITensor working using Cygwin on a laptop running Windows 10.
There are two steps, as follows:
1. Install Cygwin, making sure to put check marks for the options you want. Below I copied a write-up of the installation options I selected when I installed Cygwin:
First, download cygwin installation file, setup-x86_64.exe. It doesn't matter where it is saved.
Run the setup file. A large list of add-ons will be provided; you will need to check the following.
First click the box next to Debug, and click on the box under the column Bin? (but not Src?) for the following options:
Then click the box for the heading Devel, and select the following options:
-windwos default manifest
nano (if you want the nano editor, otherwise vi is included with gcc-core)
Some of these might be added automatically when you select the main file. For instance, gcccore, gccg++ might automatically come with cygwin_devel but I forget which ones are automatic.
If these options are not sufficient simply re-install cygwin and add more options. I found some videos on youtube that were helpful.
Cygwin should be installed within the C: drive; it will then have its' own home file, within which you should create a user folder for yourself. In this user folder, install itensor.
In this way, cygwin creates a separate computer within your computer that acts like Linux. When you run the cygwin program, for instance by clicking on the cygwin icon on your desktop if you create a desktop icon, it will open up a unix-like shell that you can run itensor from. This cygwin shell then starts you at a "home" folder that is located within cygwin; from here you can use the shell just like you would in linux with the same navigation commands.
SECOND STEP. Add the following sections in your options.mk file (I only copied the relevant parts)
Section  ------------------------------
## GCC On Windows cygwin
## Extra flags are workaround for limitations
## of this compiler with Windows binary format
CCCOM=g++ -std=c++11 -Wa,-mbig-obj -O2
## Example using a C interface to LAPACK on GNU/LINUX systems
## (Path to lib/ folder may differ on your system)
##BLAS_LAPACK_LIBFLAGS=-lpthread -L/usr/lib -lblas -llapack
BLAS_LAPACK_LIBFLAGS=-L /usr/lib -lblas -llapack
## This step is optional, but if you wish to customize the flags
## used to compile optimized and debug code, you can do so here.
## Flags to give the compiler for "release mode"
OPTIMIZATIONS=-O2 -DNDEBUG -Wall
## Flags to give the compiler for "debug mode"
DEBUGFLAGS=-DDEBUG -g -Wall -pedantic -O2
### Other Makefile variables defined for convenience.
### Not necessary to modify these for most cases.
OPTIMIZATIONS+= -DPLATFORM_$(PLATFORM) -
DEBUGFLAGS+= -DPLATFORM_$(PLATFORM) -
ITENSOR_LIBFLAGS=$(patsubst %,-l%, $(ITENSOR_LIBNAMES))
ITENSOR_LIBGFLAGS=$(patsubst %,-l%-g, $(ITENSOR_LIBNAMES))
ITENSOR_LIBS=$(patsubst %,$(ITENSOR_LIBDIR)/lib%.a, $(ITENSOR_LIBNAMES))
ITENSOR_GLIBS=$(patsubst %,$(ITENSOR_LIBDIR)/lib%-g.a, $(ITENSOR_LIBNAMES))
$(error Makefile variable CCCOM not defined in options.mk; please define it.)
CCFLAGS=-I. $(ITENSOR_INCLUDEFLAGS) $(OPTIMIZATIONS) -Wno-unused-variable
CCGFLAGS=-I. $(ITENSOR_INCLUDEFLAGS) $(DEBUGFLAGS)
Note that it took Miles a long time to figure out the compiler workarounds in the above file. But since then I have been using it with no problems.