fixes for BSD sed compatibility

[hannesm]

sorry, I wasn't carefully trying #52 on BSD.

It turns out that the expression sed -e '1i (' (and a) does not work with BSD sed:

sed: 1: "1i (
": command i expects \ followed by text

I found some explanation here, using the proposed sed -e '1 i\'$'\n''('works in a shell, but via the Makefile leads to a leading $ on a Linux (debian) system -- it works fine on FreeBSD :/

Instead, we can use sed replacements (and shell invocation) as done in this PR, and provide the parens in the .in files.

(Another solution I came up with is another conditional in Makefile:

ifeq ($(BUILD_OS),Linux)
SED_POST=
else
SED_POST=\'$$'\n''
endif

and then sed -e '1 i$(SED_POST)(' in the rules, but IMHO that one is less readable than the provided diff).

[mato]

Sorry, chaotic day today, I have to run and no time to open a PR, but how about this:

flags/libs: flags/libs.tmp Makeconf
	env PKG_CONFIG_PATH="$(shell opam config var prefix)/lib/pkgconfig" \
	    pkg-config $(PKG_CONFIG_DEPS) --libs >> $<
	awk -v RS= -- '{ \
	    sub("@@PKG_CONFIG_EXTRA_LIBS@@", "$(PKG_CONFIG_EXTRA_LIBS)", $$0); \
	    print "(", $$0, ")"\
	    }' $< >$@

#...

flags/cflags: flags/cflags.tmp Makeconf
	env PKG_CONFIG_PATH="$(shell opam config var prefix)/lib/pkgconfig" \
	    pkg-config $(PKG_CONFIG_DEPS) --cflags >> $<
	awk -v RS= -- '{ \
	    print "(", $$0, ")"\
	    }' $< >$@

Seems to do the trick for me, and the constructs work with both BSD awk and $RANDOM_LINUX awk. Also has the advantage of cleaning up newlines.

[mato]

See #57

[hannesm]

thanks for the fix!