The correct declaration is
void glGetTransformFeedbackVarying(GLuint program, GLuint index, GLsizei bufSize, GLsizei * length, GLsizei size, GLenum * type, char * name);
In glew.h, version 1.7.0 it is declared
typedef void (GLAPIENTRY * PFNGLGETTRANSFORMFEEDBACKVARYINGPROC) (GLuint, GLuint, GLint*);
Thanks for the report.
This seems fixed in top-of-tree:
$ grep PFNGLGETTRANSFORMFEEDBACKVARYINGPROC include/GL/glew.h
typedef void (GLAPIENTRY * PFNGLGETTRANSFORMFEEDBACKVARYINGPROC) (GLuint, GLuint, GLsizei, GLsizei , GLsizei , GLenum , GLchar );
Fixed in GLEW 1.9.0.
Thanks for the report.