compilation problem on Fedora 40

[elpop]

1. Install the dependencies on Fedora 40:

   sudo dnf install cmake gcc g++ LibRaw LibRaw-devel exiv2 exiv2-devel expat expat-devel libjpeg libjpeg-devel zlib zlib-devel

2. and the versions installed are:

   cmake-3.28.2-1.fc40.x86_64
   gcc-14.1.1-1.fc40.x86_64
   gcc-c++-14.1.1-1.fc40.x86_64
   LibRaw-0.21.2-5.fc40.x86_64
   LibRaw-devel-0.21.2-5.fc40.x86_64
   exiv2-0.27.6-7.fc40.x86_64
   exiv2-devel-0.27.6-7.fc40.x86_64
   expat-2.6.2-1.fc40.x86_64
   expat-devel-2.6.2-1.fc40.x86_64
   libjpeg-turbo-3.0.2-1.fc40.x86_64
   libjpeg-turbo-devel-3.0.2-1.fc40.x86_64
   zlib-ng-compat-2.1.6-2.fc40.x86_64
   zlib-ng-compat-devel-2.1.6-2.fc40.x86_64

3. use the "build" directory and use "cmake ../" and use make

i see this message in the make process:

[ 81%] Building CXX object libdng/dng-sdk/CMakeFiles/dng-sdk.dir/source/dng_string.cpp.o
/home/pop/Devel/raw2dng/libdng/dng-sdk/source/dng_string.cpp: In member function ‘void dng_string::NormalizeAsCommaSeparatedNumbers()’:
/home/pop/Devel/raw2dng/libdng/dng-sdk/source/dng_string.cpp:1917:29: error: ‘isdigit’ was not declared in this scope
 1917 |                         if (isdigit ((int) c) || c == '.' || c == '-' || c == '+' || c == 'e' || c == 'E')
      |                             ^~~~~~~
make[2]: *** [libdng/dng-sdk/CMakeFiles/dng-sdk.dir/build.make:874: libdng/dng-sdk/CMakeFiles/dng-sdk.dir/source/dng_string.cpp.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:250: libdng/dng-sdk/CMakeFiles/dng-sdk.dir/all] Error 2
make: *** [Makefile:136: all] Error 2

4. When build on Ubuntu everything is fine.