Add link-library variable to Makefiles

[noloader]

Currently the recipe to build cryptest.exe hard-codes libcryptopp.a. From GNUmakefile:

cryptest.exe:libcryptopp.a $(TESTOBJS)
	$(CXX) -o $@ $(strip $(CXXFLAGS)) $(TESTOBJS) ./libcryptopp.a $(LDFLAGS) $(LDLIBS)

Using libcryptopp.a is a good engineering decision because it sidesteps path problems that plague Unix and Linux.

However, there are two gaps. First, distros usually patch to use the shared object. Second, cryptest.sh (and some other scripts) test shared object linking, and they require makefile hacks. For example, we make a copy of the makefile, run sed over it, and then use the new makefile for testing.

We can make it easier to swap-in the shared object by introducing a makefile variable:

LINK_LIBRARY ?= ./libcryptopp.a
...

cryptest.exe: $(LINK_LIBRARY) $(TESTOBJS)
	$(CXX) -o $@ $(strip $(CXXFLAGS)) $(TESTOBJS) $(LINK_LIBRARY) $(LDFLAGS) $(LDLIBS)

This will track the changes to add the variable LINK_LIBRARY.

[noloader]

The change was added at Commit 52ad132134c1 (and friends). The change allows:

$ LINK_LIBRARY=libcryptopp.so make -j 4
...
$ LD_LIBRARY_PATH="$PWD:$LD_LIBRARY_PATH" ./cryptest.exe v
...

The user is still responsible to add a path to LDFLAGS if they don't want to use LD_LIBRARY_PATH:

$ LD_LIBRARY_PATH="$PWD:$LD_LIBRARY_PATH" ldd cryptest.exe
        linux-vdso.so.1 (0x00007ffcab9eb000)
        libcryptopp.so.8 => /home/test/libcryptopp.so.8 (0x00007fd71a067000)
        libstdc++.so.6 => /lib64/libstdc++.so.6 (0x00007fd719e55000)
        libm.so.6 => /lib64/libm.so.6 (0x00007fd719d0f000)
        libgcc_s.so.1 => /lib64/libgcc_s.so.1 (0x00007fd719cf5000)
        libpthread.so.0 => /lib64/libpthread.so.0 (0x00007fd719cd4000)
        libc.so.6 => /lib64/libc.so.6 (0x00007fd719b0e000)
        /lib64/ld-linux-x86-64.so.2 (0x00007fd71a58c000)