Homogenize paths for source files during compilation

[duthils]

Why:

• Compiling a .o file alone will compile with -c file.c
• Compiling a .lo file alone will create the corresponding .o files with -c ./file.c
• Building with make -j1 will only execute the .lo rules; the .o rules will be skipped since the .o files are already created from the .lo rules with option -c ./file.c
• Building with make -j2 will execute the .lo rules and the .o rules in parallel, creating the .o file with option -c file.c
• assert() captures the path of the source file (taken from the -c option) in the compiled binary in order to display the source of the assertion error
• Hence the compiled binaries are not reproducible depending on the number of make parallel jobs

Example:

• when compiling examples/ldns-dane with make -j1, the binary contains the string ./examples/ldns-dane.c
• when compiling examples/ldns-dane with make -j2, the binary contains the string examples/ldns-dane.c