gcc: Do not use build.with("glibc") in postinstall
gcc: Do not use build.with("glibc") in postinstall

Use Formula["glibc"].installed? rather than build.with?("glibc") to
get consistent behaviour when gcc is poured from a bottle.

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