GNU Make 4.1 breaks all our extensions making them start with ESC

[strk]

The line:

echo '\echo Use "CREATE EXTENSION postgis" to load this file. \quit' > sql/postgis.sql

In our Makefiles have started writing an ESC control character followed by "cho" as the start of all our extensions. In turn those files break creating the extension.

All branches affected, for obvious reasons.

Doubling the backslash seems to be a fix with this version of make, but I don't know how it would affect older versions. We need a better way to output those literal backslashes maybe...

[strk]

According to RhodiumToad printf(1) would be more portable, using double backslash

[strk]

In 16907:

Use portable code to write extension-protection line

References #4203 in 2.3 branch

[strk]

In 16908:

Use portable code to write extension-protection line

Closes #4203 in 2.2 branch