Install libcoloredstderr.so read-only.

Prevents accidental truncates of libcoloredstderr.so with `cp` which
crash all programs which have loaded it.

src/Makefile.am

@@ -6,3 +6,12 @@ libcoloredstderr_la_SOURCES = coloredstderr.c \
                               hookmacros.h \
                               ldpreload.h \
                               trackfds.h
+
+# Make sure the library is not writable. See README why this is important. Is
+# not run with `make libcoloredstderr.la`, but this isn't common usage.
+all-local: $(lib_LTLIBRARIES)
+	find .libs -type f -exec chmod ugo-w '{}' \;
+# `install` uses normal permissions, "fix" them so `make install` also
+# installs non-writable files.
+install-exec-hook:
+	find $(DESTDIR)$(libdir) -type f -exec chmod ugo-w '{}' \;