build: set default goal to build_src, not the first target

The Makefile had `.DEFAULT: default` intending to make the `default` target
(which builds the project) the fallback when `make` is invoked with no
arguments. `.DEFAULT` is a different special target — it provides a fallback
*recipe* for targets with no rule, and has no effect on default goal
selection.

GNU make picks the first real target as the default goal unless
`.DEFAULT_GOAL` is set. The first real target in this Makefile is
`lint-generate-cdb` (line 19), so `make` with no arguments ran
`scripts/lint/generate-compile-commands.sh`, which does
`bear -- make -j\$(nproc)` — a full rebuild of the project under Bear's
compile-intercept wrappers. Each C++ compile under Bear spawned a wrapper
process, producing hundreds of bear processes per plain `make` invocation
and burying the intended default behaviour.

Replace `.DEFAULT: default` with `.DEFAULT_GOAL := default` so `make` with
no arguments runs `default: build_src` as originally intended.

Makefile

@@ -189,7 +189,7 @@ endif
 
 ### main targets
 
-.DEFAULT: default
+.DEFAULT_GOAL := default
 .PHONY: default
 default: build_src