machines: add CFLAGS_HOST to conda_macos for recent macOS

Recent versions of Xcode and macOS do not install the system C header files
to /usr/include, but keep them in the MacOSX SDK.
Thus we need to explicitely pass their location to the compiler.

configuration/scripts/machines/Macros.conda_macos

@@ -39,6 +39,9 @@ LD:= $(FC)
 # Location of the compiled Fortran modules (NetCDF)
 MODDIR  += -I$(CONDA_PREFIX)/include
 
+# Location of the system C header files (required on recent macOS to compile makdep)
+CFLAGS_HOST += -I$(shell xcrun --show-sdk-path)/usr/include
+
 # Libraries to be passed to the linker
 SLIBS   := -lnetcdf -lnetcdff