Setting "-Ofast" in the CFLAGS for configure, then trying to make libgcrypt
results in cipher/Makefile's o_flag_munging command changing "-Ofast" (a valid
gcc flag) to "-O1fast" (an invalid gcc flag), causing the build to fail.

I worked around this by changing the line to
o_flag_munging = sed -e 's/-O([2-9s]|fast)*/-O1/g'