link error at Xcode 6.0.1

[kyfxbl]

the error message is:

Undefined symbols for architecture arm64:
"OBJC_CLASS$_EAGLContext", referenced from:
objc-class-ref in PPSSignatureView.o
"_glDisable", referenced from:
-[PPSSignatureView setupGL] in PPSSignatureView.o
"_glGenVertexArraysOES", referenced from:
-[PPSSignatureView setupGL] in PPSSignatureView.o
"_glGenBuffers", referenced from:
-[PPSSignatureView setupGL] in PPSSignatureView.o
"_glBufferData", referenced from:
-[PPSSignatureView setupGL] in PPSSignatureView.o
"_glVertexAttribPointer", referenced from:
-[PPSSignatureView bindShaderAttributes] in PPSSignatureView.o
"_glDeleteVertexArraysOES", referenced from:
-[PPSSignatureView tearDownGL] in PPSSignatureView.o
"_glMapBufferOES", referenced from:
_addVertex in PPSSignatureView.o
"_glDrawArrays", referenced from:
-[PPSSignatureView drawRect:] in PPSSignatureView.o
"_glBindBuffer", referenced from:
-[PPSSignatureView tap:] in PPSSignatureView.o
-[PPSSignatureView pan:] in PPSSignatureView.o
-[PPSSignatureView setupGL] in PPSSignatureView.o
"_glClearColor", referenced from:
-[PPSSignatureView drawRect:] in PPSSignatureView.o
"_glDeleteBuffers", referenced from:
-[PPSSignatureView tearDownGL] in PPSSignatureView.o
"_glClear", referenced from:
-[PPSSignatureView drawRect:] in PPSSignatureView.o
"_glUnmapBufferOES", referenced from:
_addVertex in PPSSignatureView.o
"_glBindVertexArrayOES", referenced from:
-[PPSSignatureView drawRect:] in PPSSignatureView.o
-[PPSSignatureView setupGL] in PPSSignatureView.o
"_glEnableVertexAttribArray", referenced from:
-[PPSSignatureView bindShaderAttributes] in PPSSignatureView.o
ld: symbol(s) not found for architecture arm64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

[MosheBerman]

Looks like you need to link GLKit. If you're using CocoaPods, this should happen automatically.