Improved mex.cmake macros get_mex_option and get_mex_arguments

* fixed = sign issue: macros weren't doing the right thing when the options or arguments lines contained a second equals sign.
* fixed todos: the macros now use CMake string parsing commands (no cut, grep or head).
* made calls to the macros faster by only calling mex -v once.

I'm not sure if the PARENT_SCOPE thing is necessary by the way; wouldn't that only be necessary if the macros were functions? They're consistent between get_mex_option and get_mex_arguments now, but maybe they should both not have the PARENT_SCOPE thing?

For reference, here's my mex -v output. Note e.g. the line containing CXXFLAGS, which contains three equals signs. The original implementation would only use arguments after the last equals sign on that line.

```
**************************************************************************
  Warning: Neither -compatibleArrayDims nor -largeArrayDims is selected. 
           Using -compatibleArrayDims. In the future, MATLAB will require
           the use of -largeArrayDims and remove the -compatibleArrayDims
           option. For more information, see:
           http://www.mathworks.com/help/matlab/matlab_external/upgrading-mex-files-to-use-64-bit-api.html
**************************************************************************

-> mexopts.sh sourced from directory (DIR = $PREF_DIR)
   FILE = /Users/twan/.matlab/R2013a/mexopts.sh
----------------------------------------------------------------
->    MATLAB                = /Applications/MATLAB_R2013a.app
->    CC                    = xcrun  -sdk macosx10.9  clang
->    CC flags:
         CFLAGS             = -fno-common -arch x86_64 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.9.sdk -mmacosx-version-min=10.9  -fexceptions
         CDEBUGFLAGS        = -g
         COPTIMFLAGS        = -O2 -DNDEBUG
         CLIBS              = -L/Applications/MATLAB_R2013a.app/bin/maci64 -lmx -lmex -lmat -lstdc++
         arguments          =  -DMX_COMPAT_32
->    CXX                   = xcrun  -sdk macosx10.9  clang++
->    CXX flags:
         CXXFLAGS           = -fno-common -fexceptions -arch x86_64 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.9.sdk -mmacosx-version-min=10.9 -std=c++11
         CXXDEBUGFLAGS      = -g
         CXXOPTIMFLAGS      = -O2 -DNDEBUG
         CXXLIBS            = -L/Applications/MATLAB_R2013a.app/bin/maci64 -lmx -lmex -lmat -lstdc++
         arguments          =  -DMX_COMPAT_32
->    FC                    = gfortran
->    FC flags:
         FFLAGS             = -fexceptions -m64 -fbackslash
         FDEBUGFLAGS        = -g
         FOPTIMFLAGS        = -O
         FLIBS              = -L/Applications/MATLAB_R2013a.app/bin/maci64 -lmx -lmex -lmat -L/usr/local/Cellar/gfortran/4.8.2/gfortran/lib/gcc/x86_64-apple-darwin13.0.0/4.8.2/../../.. -lgfortran -L/usr/local/Cellar/gfortran/4.8.2/gfortran/lib/gcc/x86_64-apple-darwin13.0.0/4.8.2 -lgfortranbegin
         arguments          =  -DMX_COMPAT_32
->    LD                    = xcrun -sdk macosx10.9 clang
->    Link flags:
         LDFLAGS            = -arch x86_64 -Wl,-syslibroot,/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.9.sdk -mmacosx-version-min=10.9 -bundle -Wl,-exported_symbols_list,/Applications/MATLAB_R2013a.app/extern/lib/maci64/mexFunction.map
         LDDEBUGFLAGS       = -g
         LDOPTIMFLAGS       = -O
         LDEXTENSION        = .mexmaci64
         arguments          = 
->    LDCXX                 = 
->    Link flags:
         LDCXXFLAGS         = 
         LDCXXDEBUGFLAGS    = 
         LDCXXOPTIMFLAGS    = 
         LDCXXEXTENSION     = 
         arguments          = 
----------------------------------------------------------------

```