Define MKDIR_P in the Makefile.in files instead in MCONFIG.in

Define MKDIR_P in the Makefile.in files instead in MCONFIG.in

In the case where mkdir -p is not thread-safe (for example, if the
build environment is using busybox's mkdir) the configure script will
fall back to the slow (but safe) install-sh script.  In that case
MKDIR_P will be using a relative pathname; so we can't use speed
optimization of defining configure substitutions in MCONFIG.in, since
the substitution will be different depending on depth of the
subdirectory in the Makefile.in file.

https://github.com/tytso/e2fsprogs/issues/51

Signed-off-by: Theodore Ts'o <tytso@mit.edu>