libindicator build system: whitespace missing before "-lm"

configure.ac sets LIBINDICATOR_LIBS+="$LIBM" , which is fine as long as the valie of LIBINICATOR_LIBS had a trailing space first. pkg-config 0.29 at least (other versions and variants are likely too) does not add a trailing space after the libs output, in the results from PKG_CHECK_MODULES. This results in the linker flags for libindicator.so having '-lglib-2.0-lm', as shown here:

https://bugs.gentoo.org/attachment.cgi?id=413560

The fix is fairly simple, just add a leading space prior to $LIBM, which should really be done every time values are appended to variables.

More info can be found at https://bugs.gentoo.org/545334