avoid escaping spaces

[t-bltg]

spaces in CYTHON_FLAGS are incorrectly escaped (at least on linux) when provided as command line arguments to the cython executable through CMake cache entries.

Here is a simple mwe (run $ bash build.sh in a terminal)
build.sh

#!/usr/bin/env bash
rm -r build; mkdir -p build; cd build

cmake \
-DCYTHON_FLAGS='-X boundscheck=False -X wraparound=False' \
-DCMAKE_MODULE_PATH="$SKBUILD_PREFIX/skbuild/resources/cmake" \
-DCMAKE_VERBOSE_MAKEFILE=ON ..

make || exit 1

python3 -c 'import fib; fib.fib(2000)'

CMakeLists.txt

cmake_minimum_required(VERSION 3.8)

project(fib)

find_package(Cython REQUIRED)
include(UseCython)

include_directories(${PYTHON_INCLUDE_DIRS})

add_cython_target(fib fib.pyx C PY3)
add_library(fib MODULE fib.c)
set_target_properties(fib PROPERTIES PREFIX "")

fib.pyx

def fib(n):
    """Print the Fibonacci series up to n."""
    a, b = 0, 1
    while b < n:
        print(b)
        a, b = b, a + b

without this patch, build.sh fails with the following output

<...>
cython --include-dir <...>/include/python3.6m -3 -X\ boundscheck=False\ -X\ wraparound=False <...>/fib.pyx --output-file <...>/build/fib.c
Error in compiler directive: boundscheck directive must be set to True or False, got 'False -X wraparound=False'
<...>
make[2]: *** [CMakeFiles/fib.dir/build.make:65: fib.c] Error 1
make[2]: Leaving directory <...>/build'

[thewtex]

@neok-m4700 Thanks for the contribution! 👏

Well done on the patch and description of the issue 🥇

[thewtex]

CI errors are unrelated.

[jcfr]

CI errors are unrelated.

These should now be fixed in #266