[doc] MAKEOVERRIDES is undocumented

[Alejandro Colomar]

Hi!

I needed to use MAKEOVERRIDES for updating a pkgconf (.pc) file.  I want it to 
have the correct directory variables as used when building/installing the 
library, so if the user specifies 'includedir=/foo/bar' in the command line, the 
.pc file should be updated with that info.

The rule I had to use is:


$(_LIB_pc): $(_LIB_pc_u) Makefile | $$(@D)/
        $(info  SED             $@)
        sed 's/Version:.*/Version: $(DISTVERSION)/' <$< >$@
        sed -i 's,prefix=.*,prefix=$(prefix),' $@
ifneq ($(filter includedir=%,$(MAKEOVERRIDES)),)
        sed -i 's,includedir=.*,includedir=$(includedir),' $@
endif


As you can see, I need to check if it was overridden in the command line 
(otherwise, the default 'includedir=${prefix}/include' in the .pc file is fine). 
 And then I do override it.

I had to search some related things in stackoverflow to find this.  I think you 
should document this (and a few others) in 'Other Special Variables' 
<https://www.gnu.org/software/make/manual/html_node/Special-Variables.html>.


Cheers,

Alex

--
<http://www.alejandro-colomar.es/>

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