musl: split out script to setup build

This makes it easier to set up a local build with musl:
$ tools/setup-musl-build.sh build-meson
$ ninja -C build-meson

.github/workflows/build-test-musl.sh

@@ -20,73 +20,5 @@ cleanup() (
 
 trap cleanup EXIT ERR INT TERM
 
-mkdir -p "${TMPDIR}/build"
-mkdir -p "${TMPDIR}/usr/include"
-
-CFLAGS="-idirafter ${TMPDIR}/usr/include"
-
-LINKS=(
-    acl
-    archive.h
-    archive_entry.h
-    asm
-    asm-generic
-    audit-records.h
-    audit_logging.h
-    bpf
-    bzlib.h
-    curl
-    dwarf.h
-    elfutils
-    fido.h
-    gcrypt.h
-    gelf.h
-    gnutls
-    gpg-error.h
-    idn2.h
-    libaudit.h
-    libcryptsetup.h
-    libelf.h
-    libkmod.h
-    linux
-    lz4.h
-    lz4frame.h
-    lz4hc.h
-    lzma
-    lzma.h
-    microhttpd.h
-    mtd
-    openssl
-    pcre2.h
-    pwquality.h
-    qrencode.h
-    seccomp-syscalls.h
-    seccomp.h
-    security
-    selinux
-    sys/acl.h
-    sys/capability.h
-    tss2
-    xen
-    xkbcommon
-    zconf.h
-    zlib.h
-    zstd.h
-    zstd_errors.h
-)
-
-for t in "${LINKS[@]}"; do
-    [[ -e /usr/include/"$t" ]]
-    link="${TMPDIR}"/usr/include/"${t}"
-    mkdir -p "${link%/*}"
-    ln -s /usr/include/"$t" "$link"
-done
-
-env \
-    CC=musl-gcc \
-    CXX=musl-gcc \
-    CFLAGS="$CFLAGS" \
-    CXXFLAGS="$CFLAGS" \
-    meson setup --werror -Ddbus-interfaces-dir=no -Dlibc=musl "${TMPDIR}"/build
-
-ninja -v -C "${TMPDIR}"/build
+tools/setup-musl-build.sh "${TMPDIR}/build"
+ninja -v -C "${TMPDIR}/build"