Makefile: Fix to work even with GZIP environment variable set.

The rule here was written to assume that if the GZIP environment
variable was set that it would be the gzip binary to execute,
(similar to the CC and CXX variables). But GZIP is actually used
to pass arguments to gzip, so we have to use a different name.

Makefile

@@ -4,7 +4,9 @@ CFLAGS=-O2
 
 # Additional programs that are used during the compilation process.
 EMACS ?= emacs
-GZIP ?= gzip
+# Lowercase to avoid clash with GZIP environment variable for passing
+# arguments to gzip.
+gzip = gzip
 
 # Additional flags that we will append to whatever the user set.
 # These aren't intended for the user to manipulate.

Makefile.local

@@ -23,7 +23,7 @@ notmuch: $(notmuch_client_modules) lib/notmuch.a
 	$(call quiet,CXX) $^ $(LDFLAGS) -o $@
 
 notmuch.1.gz: notmuch.1
-	$(call quiet,GZIP) --stdout $^ > $@
+	$(call quiet,gzip) --stdout $^ > $@
 
 install: all notmuch.1.gz
 	for d in $(DESTDIR)$(prefix)/bin/ $(DESTDIR)$(prefix)/share/man/man1 \