Update instructions how to compile FTL from source

[DL6ER]

What does this implement/fix?

Update instructions how to compile FTL from sources. Aligned with
pi-hole/FTL#2738.

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Related issue or feature (if applicable):
pi-hole/FTL#2738

Pull request in docs with documentation (if applicable): N/A

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2. I have commented my proposed changes within the code.
3. I am willing to help maintain this change if there are issues with it later.
4. It is compatible with the EUPL 1.2 license
5. I have squashed any insignificant commits. (git rebase)

Checklist:

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[sawsanders]

The download link didn't work on my setup. Ubuntu Noble.

pi@pi5:~$ wget https://github.com/Mbed-TLS/mbedtls/archive/refs/tags/v4.0.0.tar.bz2 -O mbedtls-4.0.0.tar.bz2
--2025-12-14 09:18:08--  https://github.com/Mbed-TLS/mbedtls/archive/refs/tags/v4.0.0.tar.bz2
Resolving github.com (github.com)... 140.82.116.3
Connecting to github.com (github.com)|140.82.116.3|:443... connected.
HTTP request sent, awaiting response... 404 Not Found
2025-12-14 09:18:08 ERROR 404: Not Found.

This worked however:

wget https://github.com/Mbed-TLS/mbedtls/releases/download/mbedtls-4.0.0/mbedtls-4.0.0.tar.bz2 -O mbedtls-4.0.0.tar.bz2

Also, the last compiling instruction was redundant for me.

pi@pi5:~/mbedtls-4.0.0$ sudo make install
make: *** No rule to make target 'install'.  Stop.

Running cmake --install build installed the files successfully and I was able to compile FTL with mbedtls v4.0.0:

pi@pi5:~$ pihole-FTL -vv
****************************** FTL **********************************
Version:         v6.3.3-110-g6eac8567
Branch:          testing
Commit:          6eac8567 (2025-12-14 07:03:34 -0800)
Architecture:    aarch64 (compiled locally)
Compiler:        cc (Ubuntu 13.3.0-6ubuntu2~24.04) 13.3.0
GLIBC version:   2.39

****************************** dnsmasq ******************************
Version:         pi-hole-v2.92rc1
Features:        IPv6 GNU-getopt no-DBus no-UBus no-i18n IDN2 DHCP DHCPv6 Lua TFTP no-conntrack ipset no-nftset auth DNSSEC loop-detect inotify dumpfile

****************************** SQLite3 ******************************
Version:         3.51.1
Features:        ATOMIC_INTRINSICS=1 COMPILER=gcc-13.3.0 DEFAULT_AUTOVACUUM DEFAULT_CACHE_SIZE=16384 DEFAULT_FILE_FORMAT=4 DEFAULT_FOREIGN_KEYS DEFAULT_JOURNAL_SIZE_LIMIT=-1 DEFAULT_MEMSTATUS=0 DEFAULT_MMAP_SIZE=0 DEFAULT_PAGE_SIZE=4096 DEFAULT_PCACHE_INITSZ=20 DEFAULT_RECURSIVE_TRIGGERS DEFAULT_SECTOR_SIZE=4096 DEFAULT_SYNCHRONOUS=1 DEFAULT_WAL_AUTOCHECKPOINT=1000 DEFAULT_WAL_SYNCHRONOUS=1 DEFAULT_WORKER_THREADS=0 DIRECT_OVERFLOW_READ DQS=0 ENABLE_CARRAY ENABLE_DBPAGE_VTAB ENABLE_EXPLAIN_COMMENTS ENABLE_PERCENTILE LIKE_DOESNT_MATCH_BLOBS MALLOC_SOFT_LIMIT=1024 MAX_ATTACHED=10 MAX_COLUMN=2000 MAX_COMPOUND_SELECT=500 MAX_DEFAULT_PAGE_SIZE=8192 MAX_EXPR_DEPTH=1000 MAX_FUNCTION_ARG=1000 MAX_LENGTH=1000000000 MAX_LIKE_PATTERN_LENGTH=50000 MAX_MMAP_SIZE=0x7fff0000 MAX_PAGE_COUNT=0xfffffffe MAX_PAGE_SIZE=65536 MAX_SQL_LENGTH=1000000000 MAX_TRIGGER_DEPTH=1000 MAX_VARIABLE_NUMBER=32766 MAX_VDBE_OP=250000000 MAX_WORKER_THREADS=8 MUTEX_PTHREADS OMIT_DEPRECATED OMIT_LOAD_EXTENSION OMIT_PROGRESS_CALLBACK OMIT_SHARED_CACHE SYSTEM_MALLOC TEMP_STORE=1 THREADSAFE=1

******************************** LUA ********************************
Version:         Lua 5.4.8
Libraries:       inspect.lua (8.46 KB) 

***************************** LIBNETTLE *****************************
Version:         3.10
GMP:             Full

****************************** CivetWeb *****************************
Version:         1.17 (modified by Pi-hole) with mbed TLS 4.0.0
Features:        Files: Yes, TLS: Yes, CGI: No, IPv6: Yes, 
                 WebSockets: No, Server-side JavaScript: No
                 Lua: Yes, Cache: Yes, Stats: No, Compression: No
                 HTTP2: No, Unix domain sockets: No

****************************** cJSON ********************************
Version:         1.7.18