fix gcc warning message in public header for c++11

- missing whitespace lead to msgs like:
include/bullet/LinearMath/btScalar.h:100:41:
warning: invalid suffix on literal; C++11 requires a space between
literal and string macro [-Wliteral-suffix]

- this error was visible in projects using bullet when using c++11 mode
Merge pull request #172 from tcShadowWalker/master

fix gcc warning message in public header when using c++11