Fix inconsistent pkg-config file

Some files reference modbus.pc or modbus.pc.in while other files
reference libmodbus.pc or libmodbus.pc.in.

Let stick with libmodbus.pc or libmodbus.pc.in.

Makefile.am

@@ -2,4 +2,4 @@ EXTRA_DIST = MIGRATION libmodbus.spec
 SUBDIRS = modbus tests
 
 pkgconfigdir = $(libdir)/pkgconfig
-pkgconfig_DATA = modbus.pc
+pkgconfig_DATA = libmodbus.pc

debian/libmodbus-dev.install

@@ -1,3 +1,3 @@
 debian/tmp/usr/include/modbus/*
 debian/tmp/usr/lib/libmodbus.so
-debian/tmp/usr/lib/pkgconfig/modbus.pc
+debian/tmp/usr/lib/pkgconfig/libmodbus.pc

libmodbus.spec

@@ -51,7 +51,7 @@ rm -rf $RPM_BUILD_ROOT
 %dir %{_libdir}/libmodbus.so
 %attr(0755,root,root) %{_libdir}/libmodbus.so.2.0.3
 %attr(0755,root,root) %{_libdir}/libmodbus.la
-%attr(0644,root,root) %{_libdir}/pkgconfig/modbus.pc
+%attr(0644,root,root) %{_libdir}/pkgconfig/libmodbus.pc
 %attr(0644,root,root) %{_includedir}/modbus/modbus.h
 %doc AUTHORS ChangeLog INSTALL NEWS COPYING* README