fixed link error for msvc 64 bit

How to reproduce:
 * Generate solution or ninja build with x86_64 target
 * cmake .. -G"Visual Studio 11 Win64"
 * cmake .. --build . --config Release

Error:

All ODBC targets are failed to link

Log:

4>Build started 16.10.2013 19:40:14.
2>Link:
2>     Creating library C:/WORK/GITHUB/soci/src/build.x86_64/lib/Release/soci_od
bc_3_2.lib and object C:/WORK/GITHUB/soci/src/build.x86_64/lib/Release/soci_odbc
_3_2.exp
4>InitializeBuildStatus:
4>  Touching "soci_odbc_test_mssql_static.dir\Release\soci_odbc_test_mssql_stati
c.unsuccessfulbuild".
4>CustomBuild:
4>  All outputs are up-to-date.
2>vector-use-type.obj : error LNK2001: unresolved external symbol SQLGetDiagRecA

... 

Cause:
FindODBC.cmake done wrong assumption about lib location, while MSVC compiler perfectly knows where to search for odbc32.lib required for linkage for all targets.