Improve DEFINES crossplatform compatibility in Symbian builds.

Improve DEFINES crossplatform compatibility in Symbian builds.

DEFINES statements that define strings needed separate statements for
symbian-sbsv2 builds and Makefile based plaforms, as there is one extra
layer of escaping needed in Makefile builds. Improved compatibility by
making qmake remove one layer of escaping before writing the .mmp MACROs
based on DEFINES.

Note: Symbian-abld builds still do not support string DEFINES as the
      toolchain simply can't handle them in .mmp files, no matter how
      they are escaped.

Note2: Symbian-sbsv2 support for escaped DEFINES is not perfect either,
       as bld.inf files do not like doubly escaped characters in
       extension rules (e.g. double-quotation mark as part of a string).
       This makes it impossible to pass such DEFINES to extra compilers.

Task-number: QTBUG-19232
Reviewed-by: Oswald Buddenhagen