aclocal.m4: fix == bashism in FIND_LLVM_PROG
aclocal.m4: fix == bashism in FIND_LLVM_PROG

(cherry picked from commit 0fa4240249058f327cfd283f9da2deb8dff664f1)

nathyong/microghc-ghc,forked-upstream-packages-for-ghcjs/ghc,forked-upstream-packages-for-ghcjs/ghc,forked-upstream-packages-for-ghcjs/ghc,nathyong/microghc-ghc,forked-upstream-packages-for-ghcjs/ghc,nathyong/microghc-ghc,nathyong/microghc-ghc,forked-upstream-packages-for-ghcjs/ghc,forked-upstream-packages-for-ghcjs/ghc,nathyong/microghc-ghc,nathyong/microghc-ghc,forked-upstream-packages-for-ghcjs/ghc,nathyong/microghc-ghc