Enable link-time optimization for MacVim CI builds

From local profiling, enabling LTO gives a small but measurable
improvement to performance. One test that I did was to open a really
large Markdown file with vim-markdown (which usually chokes at large
files) installed, and measure how long that takes. With LTO turned on,
usually it gives at least 6-10% performance boost, which seems
significant enough to justify turning it on as we essentially get the
improvement for free (I didn't see similar boosts in other benhcmarking
I did though, so it depends). Slight caveat is that the binary size sees
a small increase (presumably due to inlining) but it's not too much. It
takes more time to build with this turned on though, so only do this in
CI.

Author: ychin

ci/config.mk.sed

@@ -1,3 +1,6 @@
 /^CFLAGS[[:blank:]]*=/s/$/ -Wall -Wextra -Wshadow -Werror/
 /^PERL_CFLAGS_EXTRA[[:blank:]]*=/s/$/ -Wno-error=unused-function/
 /^RUBY_CFLAGS_EXTRA[[:blank:]]*=/s/$/ -Wno-error=unused-parameter/
+# Add link-time optimization for even better performance
+/^CFLAGS[[:blank:]]*=/s/$/ -flto/
+/^LDFLAGS[[:blank:]]*=/s/$/ -flto/