build/python/cmake: set CMAKE_OSX_SYSROOT on macOS

python/build/cmake.py

 import os
+import re
 import subprocess
 
 from build.project import Project
@@ -31,6 +32,26 @@ set(CMAKE_CXX_FLAGS "{toolchain.cxxflags} {toolchain.cppflags}")
     __write_cmake_compiler(f, 'C', toolchain.cc)
     __write_cmake_compiler(f, 'CXX', toolchain.cxx)
 
+    if cmake_system_name == 'Darwin':
+        # On macOS, cmake forcibly adds an "-isysroot" flag even if
+        # one is already present in the flags variable; this breaks
+        # cross-compiling for iOS, and can be worked around by setting
+        # the CMAKE_OSX_SYSROOT variable
+        # (https://cmake.org/cmake/help/latest/variable/CMAKE_OSX_SYSROOT.html).
+        m = re.search(r'-isysroot +(\S+)', toolchain.cflags)
+        if m:
+            sysroot = m.group(1)
+
+            print(f'set(CMAKE_OSX_SYSROOT {sysroot})', file=f)
+
+            # search libraries and headers only in the sysroot, not on
+            # the build host
+            f.write(f"""
+set(CMAKE_FIND_ROOT_PATH "{toolchain.install_prefix};{sysroot}")
+set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
+set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)
+""")
+
 def configure(toolchain, src, build, args=(), env=None):
     cross_args = []