That is, not only when -r was given. This allows using this veriable in mkfiles across the code base.

If there is any x86 binary in the chain of parent processes from the current one, then uname -m prints x86_64 and clang defaults to building x86_64 binaries. Detect arm64 using the kernel version and force building an arm64 toolchain instead of perpetuating x86. This is particularly important when the user shell is rc, which has been built for x86, because then rebuilding under rc continues to use x86.

Now that we assume pthreads, the only assembly left is in libmp and libsec. We only ever added assembly for 386. The portable C code is fine for plan9port.

If mk gets into a bad state, it's not obvious that you can remove the binary to force the rebuild. Also, not rebuilding means that bugs in mkmk.sh are not noticed. Just rebuild from scratch every time. It doesn't take too long compared to the rest of INSTALL.

Linux 2.4 is dead. (The libthread code hasn't worked for Linux 2.4 for a long time.)

Fixes #334.

FreeBSD 11.2 by default does not have gcc.

My MacBook Pro has hw.ncpu=12, so set NPROC=12. Speeds up INSTALL dramatically. Should probably add similar code to other OSes. Also silence rio warning from earlier commit.

Using plain 'clang' does not work well for the new devdraw on macOS 10.14. But 'xcrun --sdk macosx clang' does work, for reasons no one understands. Hopefully this will be OK on all macOS systems.

X11 headers detection, so that enable fontsrv build on OpenBSD. Change-Id: I2f3f766339a9f8beed647dfbb19a588d1825732a Reviewed-on: https://plan9port-review.googlesource.com/1290 Reviewed-by: Russ Cox <rsc@google.com>

Thanks to Akshat Kumar for reporting this issue. LGTM=seed, rsc R=rsc, seed https://codereview.appspot.com/173770043

They'll be copied back during installation but then hg doesn't have to create those files on systems that have trouble with them. TBR=rsc https://codereview.appspot.com/105800043