Augment workarounds with USE-flag oriented exceptions

[InBetweenNames]

Certain workarounds are only necessary in the presence of particular USE flags (see #75).
It would be nice to be able to specify workarounds that apply only in these cases, for example:

media-libs/mesa[nouveau] *FLAGS-=-flto*
*/ffmpeg[cpu_flags_x86_sse4_1] *FLAGS-=-flto*

This would require some patches to make it work. One approach that works right away without any patching at all is:

media-libs/mesa "has video_cards_nouveau ${IUSE//+} && use video_cards_nouveau && FlagSubAllFlags -flto*"

(courtesy of @pchome)

As contributors of this repo, I'd like to ask you what the best approach for doing this would be. I'll leave this thread open for any discussion or ideas on this.

[pchome]

The simpliest way is to add /etc/portage/bashrc.d/0000-lto-overlay-helpers.sh:

#!/bin/bash

# Helpers
# for LTO overlay [https://github.com/InBetweenNames/gentooLTO]
# and package.cflags [https://github.com/vaeth/portage-bashrc-mv]

# based on eutils.eclass:use_if_iuse()
lto_use_if_iuse() {
        [[ ${#} -eq 1 ]] || die "Invalid args to ${FUNCNAME}()"
        local flag=${1}
        local liuse=( ${IUSE} )

        has "${flag}" "${liuse[@]#[+-]}" || return 1
        use $1
}

no_lto_if_use() {
        lto_use_if_iuse $1 && FlagSubAllFlags -flto*
}

no_graphite_if_use() {
        lto_use_if_iuse $1 && FlagSubAllFlags ${GRAPHITE}
}

drop_flag_if_use() {
        [[ ${#} -eq 2 ]] || die "Invalid args to ${FUNCNAME}()"
        lto_use_if_iuse $1 && FlagSubAllFlags $2
}

...

and then media-libs/mesa "no_lto_if_use video_cards_nouveau"
or media-libs/mesa "drop_flag_if_use video_cards_nouveau -flto*"

[pchome]

Also you can simplify all this common *FLAGS-=-SGALF* with

...
noLTO() {
        FlagSubAllFlags -flto*
}

noGRAPHITE() {
        FlagSubAllFlags ${GRAPHITE}
}

fatLTO() {
        FlagAddAllFlags -ffat-lto-objects
}
...

package/name noLTO; noGRAPHITE;
package/name fatLTO;

for example