udunits2-2.2.18-1.el7 has been submitted as an update for Fedora EPEL 7.
https://admin.fedoraproject.org/updates/udunits2-2.2.18-1.el7

Package udunits2-2.2.18-1.el7:
* should fix your issue,
* was pushed to the Fedora EPEL 7 testing repository,
* should be available at your local mirror within two days.
Update it with:
# su -c 'yum update --enablerepo=epel-testing udunits2-2.2.18-1.el7'
as soon as you are able to.
Please go to the following url:
https://admin.fedoraproject.org/updates/FEDORA-EPEL-2015-1162/udunits2-2.2.18-1.el7
then log in and leave karma (feedback).

I still have the following problem, building udunits2-2.2.18-1.el7 on ppc64le epel7 mock environment:

/builddir/build/BUILD/udunits-2.2.18/build-aux/missing: line 52: makeinfo: command not found
WARNING: `makeinfo' is missing on your system.  You should only need it if
         you modified a `.texi' or `.texinfo' file, or any other file
         indirectly affecting the aspect of the manual.  The spurious
         call might also be the consequence of using a buggy `make' (AIX,
         DU, IRIX).  You might want to install the `Texinfo' package or
         the `GNU make' package.  Grab either from any GNU archive site.
make[1]: Leaving directory `/builddir/build/BUILD/udunits-2.2.18'
make[1]: *** [udunits2.html] Error 1
make: *** [install-html-recursive] Error 1

adding BuildRequires: texinfo-tex
in spec make the "makeinfo" available and the build ok.


I looked to udunits2-2.2.18-1.el7 on ppc64 and udunits2-2.2.18-1-fc22 on ppc64le successful builds and they both build without needs of texinfo package (no trace in root.log).
I guess problem is comming from the epel7 ppc64le environment but I don't have a clue of what is wrong.
I will investigate further by building on two machines and compare generated makefiles. 
Any idea of what could lead to this different behaviour ?

Seems like something is triggering a rebuild that's not necessary.  Could you post a link to the full build logs?

Created attachment 1015111 [details]
udunits2-2.2.18-1.el7 build.log

build.log file. No link as ppc64le epel7 is local environment for now.

Created attachment 1015112 [details]
udunits2-2.2.18-1.el7 root.log

root.log file. No link as ppc64le epel7 is local environment for now.

I found something strange, at the top of the build.log there is a mismatch between elf64ppc and elf64lppc.

+ export LDFLAGS
+ '[' 1 == 1 ']'
+ '[' ppc64le == ppc64le ']'
+ /usr/lib/rpm/redhat/libtool-handle-ppc64le.sh
Broken -m elf64ppc use in ./m4/libtool.m4 should handle elf64lppc.
Attempting automatic fix.
Fixed ./m4/libtool.m4 for ld -m ppc64le support.
Broken -m elf64ppc use in ./configure should handle elf64lppc.
Attempting automatic fix.
Fixed ./configure for ld -m ppc64le support.

not sure problem comes from this but I need to understand where is commong from  in epel7 ppc64le environment.

bug is not relative to this elf mismatch. Automatic fixes seems to be applied successfully.

I still suspect it may be a trigger.  It seems that something is triggering a rebuild that isn't done on other arches.  I'm also fine with adding needed BRs.

udunits2-2.2.18-1.el7 has been pushed to the Fedora EPEL 7 stable repository. If problems still persist, please make note of it in this bug report.