LDFLAGS, dependency('boost'), and werror=true cause build failures in objcpp on mac

[dankegel]

A meson.build that uses both LDFLAGS=-L/some/place (in my case, to find something with cc.find_library()) and dependency('boost') causes build errors because get_external_args() suddenly starts returning -L/some/place, which annoys clang greatly. Really, that should only show up in get_external_link_args()...

To reproduce:
On a mac os system, install homebrew, then:

$ brew install boost meson ninja
$ export LDFLAGS=-L/bogus/lib
$ echo "int main(int argc, char **argv) { return 0; }" > bug.mm
$ meson . btmp
$ cd btmp
$ ninja
clang: error: argument unused during compilation: '-L/bogus/lib' [-Werror,-Wunused-command-line-argument]

where meson.build is

project('g-speak', 'objcpp', 'cpp', default_options : [ 'werror=true'])
dependency('boost', modules : ['system'])
executable('bug', 'bug.mm')

system parameters

• plain native build
• OS is Mac OS 10.12 or 10.15, probably others as well
• Python is homebrew's 3.7.6
• meson --version 0.53.2
• ninja --version 1.10.0

Judging by https://github.com/Homebrew/homebrew-core/blob/master/Formula/meson.rb, homebrew just installs meson unpatched.