Support brace-enclosed environment variable names

This makes it possible to append alpha-numeric characters directly to an
environment variable value, e.g. ${FOO}bar will use "FOO" as the variable name
and keep the "bar" as-is, whereas $FOObar will take "FOObar" as the variable
name.
Use directories relative to current project in CheckFileOffsetBits.cmake

Enables building OpenAL Soft in a subdirectory of another project.
Use the current binary dir for shared function checks too