[CMake] Suddendly missing members when compiling on macOS Mojave

[Ruben Di Battista]

Hello, 

Tryign to compile a project that built correctly since now. I guess something changed on my system, and now I’m getting this error: 

Scanning dependencies of target Mercurve
/Applications/Xcode.app/Contents/Developer/usr/bin/make -f src/libs/CMakeFiles/Mercurve.dir/build.make src/libs/CMakeFiles/Mercurve.dir/build
[  4%] Building CXX object src/libs/CMakeFiles/Mercurve.dir/BlockMerger.cxx.o
cd /Users/rubendibattista/git/x/Mercurve/build/src/libs && /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++  -DMercurve_EXPORTS -I/Users/rubendibattista/git/x/Mercurve/src/libs -isystem /opt/local/include/vtk-8.1 -isystem /opt/local/include -isystem /opt/local/include/mpich-mp -isystem /opt/local/include/libxml2  -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk -mmacosx-version-min=10.14 -fPIC   -std=gnu++11 -o CMakeFiles/Mercurve.dir/BlockMerger.cxx.o -c /Users/rubendibattista/git/x/Mercurve/src/libs/BlockMerger.cxx
In file included from /Users/rubendibattista/git/x/Mercurve/src/libs/BlockMerger.cxx:21:
In file included from /Users/rubendibattista/git/x/Mercurve/src/libs/BlockMerger.h:28:
In file included from /Users/rubendibattista/git/x/Mercurve/src/libs/types.h:25:
In file included from /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/unordered_set:363:
In file included from /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/__hash_table:19:
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/cmath:314:9: error: no member named 'signbit' in the global namespace
using ::signbit;
      ~~^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/cmath:315:9: error: no member named 'fpclassify' in the global namespace
using ::fpclassify;
      ~~^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/cmath:316:9: error: no member named 'isfinite' in the global namespace

Something got messed up with Xcode update I guess. I also tried to downgrade Xcode without success. Does anyone have hints on how to solve this? In particular I’m not entirely sure that the `-isysroot` flag the cmake provides is correct anymore. Is that possible? 
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https://rdb.is
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