depends: Purge libtool archives

We use pkg-config where we can, which generally replaces libtool at a
higher level and does not have the same downsides as libtool. These
archives sit in our depends tree with no purpose and pollute the final
bitcoin build with massive overlinking.

depends/packages.md

@@ -151,6 +151,18 @@ Most autotools projects can be properly staged using:
 
     $(MAKE) DESTDIR=$($(package)_staging_dir) install
 
+## Build outputs:
+
+In general, the output of a depends package should not contain any libtool
+archives. Instead, the package should output `.pc` (`pkg-config`) files where
+possible.
+
+From the [Gentoo Wiki entry](https://wiki.gentoo.org/wiki/Project:Quality_Assurance/Handling_Libtool_Archives):
+
+>  Libtool pulls in all direct and indirect dependencies into the .la files it
+>  creates. This leads to massive overlinking, which is toxic to the Gentoo
+>  ecosystem, as it leads to a massive number of unnecessary rebuilds.
+
 ## Secondary dependencies:
 
 Secondary dependency packages relative to the bitcoin binaries/libraries (i.e.

depends/packages/libevent.mk

@@ -29,4 +29,5 @@ define $(package)_stage_cmds
 endef
 
 define $(package)_postprocess_cmds
+  rm lib/*.la
 endef

depends/packages/zeromq.mk

@@ -24,5 +24,5 @@ define $(package)_stage_cmds
 endef
 
 define $(package)_postprocess_cmds
-  rm -rf bin share
+  rm -rf bin share lib/*.la
 endef