build: LDFLAGS are not honored during libimpulse generation

[mmitch]

The Code Coverage build workflow needs some extra flags to enable code coverage statistics.
These flags don't work with the generation of impulse.h (LDFLAGS seems to be ignored), so the workflow currently adopts a two-step build:

gbsplay/.github/workflows/code_coverage.yml

Lines 22 to 39 in 5e83317

	- name: Build impulsegen
	# this builds impulsegen without coverage - LDFLAGS are not honored during build, so it would fail otherwise :/
	run: |
	./configure
	make impulse.h
	- name: Build with coverage
	# this builds gbsplay with coverage enabled with GCC
	env:
	CFLAGS: '-O0 -coverage'
	LDFLAGS: '-lgcov -coverage'
	run: |
	./configure
	make gbsplay
	- name: Test with coverage
	# run the test under coverage
	run: |
	./gbsplay -c examples/gbsplayrc_sample -E b -o stdout examples/nightmode.gbs >/dev/null
	gcov *.o

1. impulse.h is generated with default flags, thus impulsegen.c is not considered covered code
2. the remaining build is run again, this time with coverage enabled

Can we make this work in a single pass without changing the build flags in the middle?