Not all environments have 'diff' installed. Most notably CentOS 8
does not have diff out-of-the-box. Thus users running 'cmake .' and
'make' would fail to build MariaDB, and they would think the error
was in ABI incompatibilities due to the error message emitted by CMake
when in reality simply 'diff' was missing.

This fixes it and makes the developer experience better by simply skipping
the diffing if 'diff' is not found.

## Proof

Running in a clean CentOS 8 container.

### Before

```
$ cmake .
...
-- Looking for cpuid.h
-- Looking for cpuid.h - found
-- Looking for x86intrin.h
-- Looking for x86intrin.h - found
-- Looking for event.h
-- Looking for event.h - not found
-- Configuring done
-- Generating done
-- Build files have been written to: /build

$ make
Scanning dependencies of target abi_check
CMake Error at cmake/do_abi_check.cmake:82 (MESSAGE):
  ABI check found difference between /build/include/mysql/plugin_audit.h.pp
  and /build/abi_check.out

make[2]: *** [CMakeFiles/abi_check.dir/build.make:57: CMakeFiles/abi_check] Error 1
make[1]: *** [CMakeFiles/Makefile2:168: CMakeFiles/abi_check.dir/all] Error 2
make: *** [Makefile:163: all] Error 2

$ yum install -y diffutils
...
Installed:
  diffutils-3.6-6.el8.x86_64
Complete!

$ make
[  0%] Built target abi_check
Scanning dependencies of target INFO_BIN
[  0%] Built target INFO_BIN
Scanning dependencies of target INFO_SRC
[  0%] Built target INFO_SRC
Scanning dependencies of target wsrep_api_v26
[  0%] Building C object wsrep-lib/wsrep-API/CMakeFiles/wsrep_api_v26.dir/v26/wsrep_dummy.c.o
[  0%] Building C object wsrep-lib/wsrep-API/CMakeFiles/wsrep_api_v26.dir/v26/wsrep_gtid.c.o
[  0%] Building C object wsrep-lib/wsrep-API/CMakeFiles/wsrep_api_v26.dir/v26/wsrep_loader.c.o
[  0%] Building C object wsrep-lib/wsrep-API/CMakeFiles/wsrep_api_v26.dir/v26/wsrep_uuid.c.o
[  0%] Linking C static library libwsrep_api_v26.a
[  0%] Built target wsrep_api_v26
```

### After

```
$ make
Command 'diff' not found. ABI check for /build/server/include/mysql/plugin_audit.h skipped.
Command 'diff' not found. ABI check for /build/server/include/mysql/plugin_ftparser.h skipped.
Command 'diff' not found. ABI check for /build/server/include/mysql.h skipped.
Command 'diff' not found. ABI check for /build/server/include/mysql/psi/psi_abi_v1.h skipped.
Command 'diff' not found. ABI check for /build/server/include/mysql/psi/psi_abi_v2.h skipped.
Command 'diff' not found. ABI check for /build/server/include/mysql/client_plugin.h skipped.
Command 'diff' not found. ABI check for /build/server/include/mysql/plugin_auth.h skipped.
Command 'diff' not found. ABI check for /build/server/include/mysql/plugin_password_validation.h skipped.
Command 'diff' not found. ABI check for /build/server/include/mysql/plugin_encryption.h skipped.
Command 'diff' not found. ABI check for /build/server/include/mysql/plugin_data_type.h skipped.
Command 'diff' not found. ABI check for /build/server/include/mysql/plugin_function.h skipped.
[  0%] Built target abi_check
[  0%] Built target INFO_SRC
[  0%] Built target INFO_BIN
[  0%] Built target wsrep_api_v26
[  0%] Building CXX object wsrep-lib/src/CMakeFiles/wsrep-lib.dir/server_state.cpp.o
```

If diff is installed, those warnings are simply not shown. Builds pass
without the need to install 'diff'.