qwt: Fix for Linuxbrew
qwt: Fix for Linuxbrew

`qwt` was appending Mac-specific `qmake` flags.

Closes Linuxbrew/linuxbrew#927.
Closes Linuxbrew/linuxbrew#936.

Signed-off-by: Shaun Jackman <b580dab3251a9622aba3803114310c23fdb42900@gmail.com>

SuperNEMO-DBD/cadfaelbrew,saketkc/linuxbrew,SuperNEMO-DBD/cadfaelbrew,Homebrew/linuxbrew,Linuxbrew/linuxbrew,Homebrew/linuxbrew,gnubila-france/linuxbrew,sjackman/linuxbrew,Angeldude/linuxbrew,saketkc/linuxbrew,sjackman/linuxbrew,SuperNEMO-DBD/cadfaelbrew,Linuxbrew/linuxbrew,Homebrew/linuxbrew,Angeldude/linuxbrew,Angeldude/linuxbrew,gnubila-france/linuxbrew,Linuxbrew/linuxbrew,reelsense/linuxbrew,Linuxbrew/linuxbrew,saketkc/linuxbrew,Homebrew/linuxbrew,gnubila-france/linuxbrew,SuperNEMO-DBD/cadfaelbrew,Angeldude/linuxbrew,reelsense/linuxbrew,reelsense/linuxbrew,sjackman/linuxbrew,sjackman/linuxbrew,reelsense/linuxbrew,gnubila-france/linuxbrew,saketkc/linuxbrew