Not sure if this is a dream, but what if in the PKGBUILD you could do something like:
remove_makeflag '-j'
and it would leave everything else intact except for the '-j5' flag I have set, for instance? We could do something similar with cflags and ldflags:
remove_ldflag '--as-needed'
remove_cflag '-O2' (this would touch both cflags and cxxflags)

Please set me in my place if you think this is going too far.

I actually think this is a good idea. Most of the issues I come across with {C,LD}FLAGS are due to a single flag. But there are cases where you want to disable CFLAGS completely and use upstream defaults (firefox) so as long as both can occur, I would be happy.

Doesn't firefox use its own cflags by default anyway ? I remember an option about cflags in the build config file.
Anyway it does not matter.

If you want to disable CFLAGS completely, maybe that's easy enough to do directly in the pkgbuild ? Like unset CFLAGS or CFLAGS="" ?
What seems more tricky is to remove something.

What if you want to remove "-j[ ]*[0-9]*" or "-O[ ]*[0-9]*" , that is any -j or -O settings ?

remove_makeflag '-j'
remove_cflag '-O'

I still don't clearly see how you can know when you want to transform the argument to a regexp (-O , -j) , and when you don't (-O2, --as-needed, etc)

patch: http://mailman.archlinux.org/pipermail/pacman-dev/2010-December/012132.html

This just implements complete clearing of CFLAGS, CXXFLAGS and LDFLAGS which is primarily useful for testing build issues. Clearing of individual flags seems too difficult to do properly.