Do not hardcode -lstdc++ (symbol mismatch with libc++abi and libestdc++).

Index: libraw.pc.in
--- libraw.pc.in.orig
+++ libraw.pc.in
@@ -7,6 +7,6 @@ Name: libraw
 Description: Raw image decoder library (non-thread-safe)
 Requires: @PACKAGE_REQUIRES@
 Version: @PACKAGE_VERSION@
-Libs: -L${libdir} -lraw -lstdc++@PC_OPENMP@
+Libs: -L${libdir} -lraw@PC_OPENMP@
 Libs.private: @PACKAGE_LIBS_PRIVATE@
 Cflags: -I${includedir}/libraw -I${includedir}