That should be only on the CFlags. When using the compiler as the linker driver, one may want to also call pkg-config to get the cflags, but when invoking the linker, there should be no compiler flags.
@@ -6,6 +6,6 @@ includedir=@includedir@
Name: Protocol Buffers
Description: Google's Data Interchange Format
Version: @VERSION@
-Libs: -L${libdir} -lprotobuf-lite @PTHREAD_CFLAGS@ @PTHREAD_LIBS@
+Libs: -L${libdir} -lprotobuf-lite @PTHREAD_LIBS@
Cflags: -I${includedir} @PTHREAD_CFLAGS@
Conflicts: protobuf
@@ -6,7 +6,7 @@ includedir=@includedir@
-Libs: -L${libdir} -lprotobuf @PTHREAD_CFLAGS@ @PTHREAD_LIBS@
+Libs: -L${libdir} -lprotobuf @PTHREAD_LIBS@
Libs.private: @LIBS@
Cflags: -I${includedir} @PTHREAD_CFLAGS@ @CXXFLAGS@
Conflicts: protobuf-lite