auto/make: Make GNU make mandatory

There is breakage on non-GNU make systems due to the OTEL stuff using
'ifeq'.

We had previously handled this by only using such constructs (and
enabling certain features, such as D= & E=) when using GNU make.

OTEL added this unconditionally. It's used to set the target build
directory appropriately depending on whether we are doing debug or
release builds.

Unfortunately it seems there is no way to control this via cargo/rustc
command line arguments or environment variables.

For BSD make we could use '.if' etc but then trying to handle these two
systems (and possibly more) in the build scripts is going to be
unwieldy.

Let's just bite the bullet and make (no pun intended) GNU make mandatory
to build Unit, it's the default on Linux (I'm sure there's an exception
somewhere...) and it seems the default on macOS. On other systems it's
readily available via gmake(1).

This also brings build feature parity to all systems out the box.

As it happens, this also fixes issues with using gmake...

Fixes: 9d3dcb8 ("otel: add build tooling to include otel code")
Link: <#1520 (comment)>
Signed-off-by: Andrew Clayton <[email protected]>

auto/make

@@ -36,14 +36,7 @@ manpage: $NXT_BUILD_DIR/share/man/man8/unitd.8
 
 END
 
-
-NXT_OS=$(uname -s)
-NXT_GNU_MAKE=$(make --version | grep GNU || true)
-
-# Requires GNU make. On OpenIndiana at least we have to use gmake
-if [ -n "$NXT_GNU_MAKE" ] || [ $NXT_OS = "SunOS" ]; then
-
-	cat << END >> $NXT_MAKEFILE
+cat << END >> $NXT_MAKEFILE
 # By default compiler etc output is hidden, use
 #   make V=1 ...
 # to show it.
@@ -78,10 +71,9 @@ endif
 
 END
 
-fi
-
 # potentially set otel lib location
 if [ $NXT_OTEL = YES ]; then
+
 cat << END >> $NXT_MAKEFILE
 
 ifeq (\$D,1)
@@ -91,6 +83,7 @@ else
 endif
 
 END
+
 fi
 
 # The include paths list.