hamlib.pc.in: Changes conformant to pkg-config docs

A careful reading of the pkg-config user guide and manual page showed
some errors in the format of the generated hamlib.pc file.  This patch,
hopefully, corrects those errors.

hamlib.pc.in

@@ -3,10 +3,12 @@ exec_prefix=${prefix}
 libdir=${exec_prefix}/lib
 includedir=${prefix}/include
 
-Name: hamlib
+Name: @PACKAGE_NAME@
 Description: Library to control radio and rotator equipment.
 URL: @PACKAGE_URL@
-Requires: 
-Version: @VERSION@
-Libs: -L${libdir} -lhamlib @MATH_LIBS@
+Version: @PACKAGE_VERSION@
+Requires.private: libusb >= 0.1
+Conflicts: libusb >= 1.0
 Cflags: -I${includedir}
+Libs: -L${libdir} -lhamlib
+Libs.private: -lm -ldl -lpthread