build fails with icu 75

[andyrtr]

Building with icu 75.1 fails:

[ 80%] Building CXX object CMakeFiles/slopy.dir/src/glrectangle.cpp.o
[ 86%] Linking CXX shared library libslopy.so
[ 86%] Built target slopy
[ 93%] Building CXX object CMakeFiles/slop.dir/src/main.cpp.o
In file included from /usr/include/unicode/unistr.h:39,
                 from /build/slop/src/slop-7.6/src/cxxopts.hpp:51,
                 from /build/slop/src/slop-7.6/src/main.cpp:28:
/usr/include/unicode/stringpiece.h:133:29: error: ‘enable_if_t’ in namespace ‘std’ does not name a template type
  133 |             typename = std::enable_if_t<
      |                             ^~~~~~~~~~~
/usr/include/unicode/stringpiece.h:133:24: note: ‘std::enable_if_t’ is only available from C++14 onwards
  133 |             typename = std::enable_if_t<
      |                        ^~~
/usr/include/unicode/stringpiece.h:133:40: error: expected ‘>’ before ‘<’ token
  133 |             typename = std::enable_if_t<
      |                                        ^
make[2]: *** [CMakeFiles/slop.dir/build.make:76: CMakeFiles/slop.dir/src/main.cpp.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:111: CMakeFiles/slop.dir/all] Error 2
make: *** [Makefile:136: all] Error 2