CMake toolchain not finding standard C/C++ includes or libraries - c++

Question

I'm trying to write my CMakeLists.txt files in a way that can be compiled with and without toolchain support, but I am running into compile and linker issues where common C/C++ libraries aren't getting found or linked.
After reading CMake toolchain documentation the only configuration option I could see that would control how the standard libraries are found is the CMAKE_SYSROOT variable.
I happen to be using a TI toolchain so I specify a path set(CMAKE_SYSROOT "C:/ti/ccs1031/ccs/tools/compiler/ti-cgt-arm_20.2.4.LTS")
When I compile without specifying a toolchain file everything works fine and I get the compiled target for the host system.
> cmake -E make_directory build && \
> cmake -E chdir build cmake .. && \
> cmake --build build --config Debug
-- Selecting Windows SDK version 10.0.18362.0 to target Windows 10.0.19042.
-- Configuring done
-- Generating done
-- Build files have been written to: C:/src/cmake-toolchain-example/build
Microsoft (R) Build Engine version 17.0.0+c9eb9dd64 for .NET Framework
Copyright (C) Microsoft Corporation. All rights reserved.
Checking Build System
Building Custom Rule C:/src/cmake-toolchain-example/CMakeLists.txt
example.vcxproj -> C:\src\cmake-toolchain-example\build\Debug\example.exe
Building Custom Rule C:/src/cmake-toolchain-example/CMakeLists.txt
However, when I try compiling with the toolchain that uses the TI toolchain I get errors related to standard library.
> cmake -E make_directory ../build && \
> cmake -E chdir ../build cmake -G "Eclipse CDT4 - Ninja" --toolchain cmake/Toolchains/ti-cgt-toolchain.cmake ../cmake-toolchain-example && \
> cmake --build ../build --config Debug
-- Using toolchain file: cmake/Toolchains/ti-cgt-toolchain.cmake
-- Using toolchain file: C:/src/cmake-toolchain-example/cmake/Toolchains/ti-cgt-toolchain.cmake
-- The C compiler identification is TI 20.2.4
-- The CXX compiler identification is TI 20.2.4
-- Found Eclipse version 4.14 ()
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - failed
-- Detecting C compile features
-- Detecting C compile features - failed
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - failed
-- Detecting CXX compile features
-- Detecting CXX compile features - failed
-- Configuring done
-- Generating done
-- Build files have been written to: C:/src/build
[1/2] Building CXX object CMakeFiles\example.dir\main.cpp.obj
FAILED: CMakeFiles/example.dir/main.cpp.obj
C:\ti\ccs1031\ccs\tools\compiler\ti-cgt-arm_20.2.4.LTS\bin\armcl.exe --compile_only --cpp_file=C:\src\cmake-toolchain-example\main.cpp --preproc_with_compile --preproc_dependency=CMakeFiles\example.dir\main.cpp.obj.d --output_file=CMakeFiles\example.dir\main.cpp.obj
"C:\src\cmake-toolchain-example\main.cpp", line 1: fatal error: could not open source file "iostream" (no directories in search list)
1 catastrophic error detected in the compilation of "C:\src\cmake-toolchain-example\main.cpp".
Compilation terminated.
>> Compilation failure
ninja: build stopped: subcommand failed.
I think I am missing something obvious when it comes to crosscompiling that is automatically handled when using the default CMake generators.
When using toolchain files, does one have to manually add the include/, lib/, etc to their respective CMake variables?
I guess I was under the impression that the CMAKE_SYSROOT along with other CMake variables such as CMAKE_SYSTEM_PREFIX_PATH, CMAKE_LIBRARY_PATH, CMAKE_SYSTEM_INCLUDE_PATH, etc use some sane defaults that work for most toolchains.
The standard C/C++ includes for my toolchain are located at C:\ti\ccs1031\ccs\tools\compiler\ti-cgt-arm_20.2.4.LTS\include and there is a libc.a that is available at C:\ti\ccs1031\ccs\tools\compiler\ti-cgt-arm_20.2.4.LTS\lib.
However, since I don't have to specify that standard stuff in my CMakeLists.txt when not using a toolchain file I would like to not specify it when I am using a toolchain file.
What else is needed in the toolchain file to help resolve C/C++ standard library issues like this?
I will provide a simple git repo to demonstrate this issue located here.
EDIT:
I came across this e2e forum post that was pretty helpful.