Fix build on Linux and OSX

[PaGrom]

There were 2 problems:

1. (Affected OSX and Linux) Wrong path to "packages.config". Xbuild tried to search it in SolutionDir, not in ProjectDir.
2. (Affected Linux) Linux has case-sensitive file system, so xbuild couldn't start "nuget.exe"

Command for build with mono:
EnableNuGetPackageRestore=true MONO_IOMAP=all xbuild build.proj /target:Build /p:Configuration="Release 4.0" /p:Platform="Any CPU"

[PaGrom]

Add Jira-issue http://dev.clojure.org/jira/browse/CLJCLR-70

[dmiller]

Prefer patches on JIRA.
Need to have signed the Clojure Contributor's Agreement.
These fixes are straightforward and have bee made.