cons: fix for multiple include dirs returned by xml2-config

[plexoos]

xml2-config --cflags may return multiple paths in which case cons
expects them to be separated by ':'

[plexoos]

Here are some more details on what's being fixed and why. Building on a rcas node with the STAR's csh login scripts failed for me with the following error:

$ starver dev config/v0.2.3-rhel7-root5.34.38-32b
$ cons +StDbLib
....
g++ -m32 -fPIC -pipe -Wall -Woverloaded-virtual -std=c++0x -Wno-long-long -pthread -Wno-deprecated-declarations -g -Dsl73_gcc485 -D__ROOT__ -DNEW_DAQ_READER -I. -IStRoot -I.sl73_gcc485/include -I/afs/rhic.bnl.gov/star/packages/DEV -I/afs/rhic.bnl.gov/star/packages/DEV/StRoot -I/afs/rhic.bnl.gov/star/packages/DEV/.sl73_gcc485/include -I/cvmfs/star.sdcc.bnl.gov/star-spack/spack/opt/spack/linux-rhel7-x86/gcc-4.8.5/root-5.34.38-iohvmc4ws6ykoeqo26brhlaf5fdeq5jz/include -I/cvmfs/star.sdcc.bnl.gov/star-spack/spack/opt/spack/linux-rhel7-x86/gcc-4.8.5/mysql-5.6.43-l53d4b6d4hx44oqovuc27nnetbjp2ybo/include -c .sl73_gcc485/obj/StRoot/StDbLib/MysqlDb.cc -o .sl73_gcc485/obj/StRoot/StDbLib/MysqlDb.o
In file included from .sl73_gcc485/obj/StRoot/StDbLib/StDbServiceBroker.h:22:0,
                 from .sl73_gcc485/obj/StRoot/StDbLib/StDbManagerImpl.hh:72,
                 from .sl73_gcc485/obj/StRoot/StDbLib/MysqlDb.h:154,
                 from .sl73_gcc485/obj/StRoot/StDbLib/MysqlDb.cc:272:
.sl73_gcc485/obj/StRoot/StDbLib/StlXmlTree.h:3:30: fatal error: libxml/xmlreader.h: No such file or directory
 #include <libxml/xmlreader.h>
                              ^
compilation terminated.

Apparently, the include dirs for libxml2 are missing, although the build log shows the expected value for $XMLINCDIR:

#10 1.017 Using /opt/software/linux-scientific7-x86_64/gcc-4.8.5/libxml2-2.9.13-bcnwcywvldxfk3lgq2kosswuuomjr3fn/bin/xml2-config
#10 1.017 	XMLLIBDIR = /opt/software/linux-scientific7-x86_64/gcc-4.8.5/libxml2-2.9.13-bcnwcywvldxfk3lgq2kosswuuomjr3fn/lib
#10 1.017 	XMLINCDIR = /opt/software/linux-scientific7-x86_64/gcc-4.8.5/libxml2-2.9.13-bcnwcywvldxfk3lgq2kosswuuomjr3fn/include/libxml2 -I/opt/software/linux-scientific7-x86_64/gcc-4.8.5/libiconv-1.16-fdvezl2dsxavxucpwqvya2klfxlu7na2/include
#10 1.017 	XMLLIBS = -lxml2 -L/opt/software/linux-scientific7-x86_64/gcc-4.8.5/zlib-1.2.12-4z3ybgoz3vlkmxd4mk37grmzbuoifusb/lib -lz -L/opt/software/linux-scientific7-x86_64/gcc-4.8.5/xz-5.2.5-2edpl3mylaok7qd3i4u3hw33rtkstoqp/lib -llzma -L/opt/software/linux-scientific7-x86_64/gcc-4.8.5/libiconv-1.16-fdvezl2dsxavxucpwqvya2klfxlu7na2/lib -liconv -lm -ldl
#10 1.017 	XMLCPPFlag = 

This is similar to the output in the containers but it does not cause the same problem. Not sure if it matters here but in the containers we use bash and an older version of perl...

After the fix the code can be compiled and the cons' output looks like this:

#10 1.276 Using /opt/software/linux-scientific7-x86_64/gcc-4.8.5/libxml2-2.9.13-bcnwcywvldxfk3lgq2kosswuuomjr3fn/bin/xml2-config
#10 1.276 	XMLLIBDIR = /opt/software/linux-scientific7-x86_64/gcc-4.8.5/libxml2-2.9.13-bcnwcywvldxfk3lgq2kosswuuomjr3fn/lib
#10 1.276 	XMLINCDIR = /opt/software/linux-scientific7-x86_64/gcc-4.8.5/libxml2-2.9.13-bcnwcywvldxfk3lgq2kosswuuomjr3fn/include/libxml2:/opt/software/linux-scientific7-x86_64/gcc-4.8.5/libiconv-1.16-fdvezl2dsxavxucpwqvya2klfxlu7na2/include
#10 1.276 	XMLLIBS = -lxml2 -L/opt/software/linux-scientific7-x86_64/gcc-4.8.5/zlib-1.2.12-4z3ybgoz3vlkmxd4mk37grmzbuoifusb/lib -lz -L/opt/software/linux-scientific7-x86_64/gcc-4.8.5/xz-5.2.5-2edpl3mylaok7qd3i4u3hw33rtkstoqp/lib -llzma -L/opt/software/linux-scientific7-x86_64/gcc-4.8.5/libiconv-1.16-fdvezl2dsxavxucpwqvya2klfxlu7na2/lib -liconv -lm -ldl
#10 1.276 	XMLCPPFlag =