Makefile: enable static compilation with musl

Assuming a Debian GNU/Linux system with wget, tar, git, make
and musl-gcc

```
mkdir build
cd build
export BUILD_TARGET=$PWD
export CC="musl-gcc"

echo download and build zlib
wget http://zlib.net/current/zlib.tar.gz
tar xvf zlib.tar.gz
cd zlib-1.3.1/
./configure --prefix=$BUILD_TARGET --static
make
make install
cd ..

echo download and build openssl
wget https://github.com/openssl/openssl/releases/download/openssl-3.4.0/openssl-3.4.0.tar.gz
tar xvf openssl-3.4.0.tar.gz
cd openssl-3.4.0
CC="musl-gcc -fPIE -pie -static -idirafter /usr/include/ -idirafter /usr/include/x86_64-linux-gnu/" ./Configure no-shared no-async --prefix=$BUILD_TARGET --openssldir=$BUILD_TARGET/ssl linux-x86_64
make depend
make
make install
cd ..

echo download and build curl
wget https://curl.se/download/curl-7.88.1.tar.gz
tar xvf curl-7.88.1.tar.gz
cd curl-7.88.1
./configure --disable-shared --enable-static --disable-silent-rules --disable-debug --disable-warnings --disable-werror --disable-curldebug --disable-symbol-hiding --disable-ares --disable-rt --disable-ech --disable-dependency-tracking --disable-libtool-lock --enable-http --disable-ftp --disable-file --disable-ldap --disable-ldaps --disable-rtsp --disable-proxy --disable-dict --disable-telnet --disable-tftp --disable-pop3 --disable-imap --disable-smb --disable-smtp --disable-gopher --disable-mqtt --disable-manual --disable-libcurl-option --disable-ipv6 --disable-openssl-auto-load-config --disable-versioned-symbols --disable-verbose --disable-sspi --disable-crypto-auth --disable-ntlm --disable-ntlm-wb --disable-tls-srp --enable-unix-sockets --disable-cookies --disable-socketpair --disable-http-auth --disable-doh --disable-mime --disable-dateparse --disable-netrc --disable-progress-meter --disable-dnsshuffle --disable-get-easy-options --disable-alt-svc --disable-websockets --without-brotli --without-zstd --without-libpsl --without-libgsasl --without-librtmp --without-winidn --with-openssl=$BUILD_TARGET/ --disable-threaded-resolver --with-zlib=$BUILD_TARGET/ --prefix=$BUILD_TARGET/
make
make install
cd ..

echo download and build snac2
git clone https://codeberg.org/grunfink/snac2.git
cd snac2 # assuming this commit in master
make CFLAGS="-g -Wall -Wextra -pedantic -static -DWITHOUT_SHM" PREFIX=$BUILD_TARGET LDFLAGS="-L$BUILD_TARGET/lib64 -lcurl -lssl -lcrypto -lz"
make install PREFIX=$BUILD_TARGET
cd ..

echo a statically linked snac is ready at $BUILD_TARGET/bin
```

Makefile

@@ -1,4 +1,4 @@
-PREFIX=/usr/local
+PREFIX?=/usr/local
 PREFIX_MAN=$(PREFIX)/man
 CFLAGS?=-g -Wall -Wextra -pedantic
 
@@ -6,16 +6,16 @@ all: snac
 
 snac: snac.o main.o data.o http.o httpd.o webfinger.o \
     activitypub.o html.o utils.o format.o upgrade.o mastoapi.o
-	$(CC) $(CFLAGS) -L/usr/local/lib *.o -lcurl -lcrypto $(LDFLAGS) -pthread -o $@
+	$(CC) $(CFLAGS) -L$(PREFIX)/lib *.o -lcurl -lcrypto $(LDFLAGS) -pthread -o $@
 
 .c.o:
-	$(CC) $(CFLAGS) $(CPPFLAGS) -I/usr/local/include -c $<
+	$(CC) $(CFLAGS) $(CPPFLAGS) -I$(PREFIX)/include -c $<
 
 clean:
 	rm -rf *.o *.core snac makefile.depend
 
 dep:
-	$(CC) -I/usr/local/include -MM *.c > makefile.depend
+	$(CC) -I$(PREFIX)/include -MM *.c > makefile.depend
 
 install:
 	mkdir -p -m 755 $(PREFIX)/bin