fix partial updates to libgray.a when using Nix

When building the static library the Nixpkgs stdenv `ar` defaults to not
writing timestamps for the members file. This imply make will always try
to add all members to the archive, which defeats the purpose of the
file-level granularity.

Makefile

@@ -69,6 +69,13 @@ LD        = gfortran
 FFLAGS   += -J$(OBJDIR) -I$(INCDIR) -ffree-line-length-none -fPIC
 CPPFLAGS += -DREVISION=\"$(GIT_REV)$(GIT_DIRTY)\" -DPREFIX=\"$(PREFIX)\"
 
+ifndef DETERMINISTIC
+ifdef AR_DEFAULT_DETERMINISTIC
+# Write timestamps to allow partial updates
+ARFLAGS = crU
+endif
+endif
+
 # Static build options
 ifdef STATIC
 LIBRARIES := $(LIBRARIES:so=a)
@@ -109,7 +116,7 @@ install: $(BINARIES) $(LIBRARIES) $(SHAREDIR)/doc $(SHAREDIR)/gray.1
 $(OBJDIR)/%.o: $(OBJDIR)/%.d
 
 # GRAY binary
-$(GRAY): $(shell ./depend $(OBJDIR)/main.o) | $(BINDIR)
+$(GRAY): $(OBJDIR)/main.o $(LIBGRAY).a | $(BINDIR)
 	$(LD) $(LDFLAGS) -o '$@' $^
 
 # GRAY shared library

configure

@@ -15,6 +15,7 @@ Options:
   --prefix=PREFIX         install files in PREFIX [/usr/local]
   --enable-static         statically link programs and libraries [no]
   --disable-static        dynamically link programs and libraries [yes]
+  --enable-deterministic  try to make a bit-for-bit deterministic build [no]
   --with-katex=PATH       specify path to the KaTeX library [automatically downloaded]
 EOF
 }
@@ -40,6 +41,7 @@ for arg in "$@"; do
       ;;
     --enable-static) printf 'STATIC=1\n' >> configure.mk ;;
     --disable-static) ;;
+    --enable-deterministic) printf 'DETERMINISTIC=1\n' >> configure.mk ;;
     --with-katex=*) printf 'KATEX_URL==file://%s/\n' "${arg#*=}" >> configure.mk ;;
     *=*)
       printf '%s\n' "${arg?}" >> configure.mk
@@ -79,3 +81,8 @@ if [ $? -ne 0 ]; then
 fi
 printf 'using %s as Fortran compiler\n' "$FC"
 printf '%s=%s\n' FC "$FC" >> configure.mk
+
+# Check whether ar is deterministic by default
+if ar h | grep -q '\[D\].*(default)' 2>/dev/null; then
+  printf 'AR_DEFAULT_DETERMINISTIC=1\n' >> configure.mk
+fi

default.nix

@@ -59,6 +59,7 @@ in
     configureFlags = [
       (lib.enableFeature static "static")
       "--with-katex=${katex}"
+      "--enable-deterministic"
       "GIT_REV=${version}"
       "GIT_DIRTY="
     ];