cmake does not provide 'LIBSUFFIX' variables, but provides 'LIB_SUFFIX'.
https://bugs.gentoo.org/730722
--- a/capstone.pc.in
+++ b/capstone.pc.in
@@ -1,6 +1,6 @@
 prefix=@CMAKE_INSTALL_PREFIX@
 exec_prefix=${prefix}
-libdir=${prefix}/lib@LIBSUFFIX@
+libdir=${prefix}/@CMAKE_INSTALL_LIBDIR@
 includedir=${prefix}/include/capstone
 
 Name: capstone