Without `-isysroot <SDK_PATH>` the build fails:

```
/Library/Developer/CommandLineTools/usr/bin/make[4]: Making `all' in   \
    `makeconv'
/Library/Developer/CommandLineTools/usr/bin/c++ -O2 -W -Wall -pedantic \
    -Wpointer-arith -Wwrite-strings -Wno-long-long -std=c++11          \
    -Wno-ambiguous-reversed-operator     -o ../../bin/makeconv         \
    gencnvex.o genmbcs.o makeconv.o ucnvstat.o -L../../lib -licutu     \
    -L../../lib -licui18n -L../../lib -licuuc -L../../stubdata         \
    -licudata -lpthread -lm
ld: library not found for -lpthread
clang: error: linker command failed with exit code 1 (use -v to see    \
    invocation)
```

ICU is written on C++, so we should pass `CXXFLAGS`, not only `CFLAGS`.

Note: `CPPFLAGS` stands for C preprocessor flags, not C++ flags.

Fixes #7459

NO_DOC=it fixes a build failure for one of build types, nothing to
       document
NO_TEST=it will be tested in a next commit by enabling corresponding
        GitHub Action workflow