build fails on darwin without brew installed

build fails due to brew command missing on systems like nixpkgs on darwin when it would otherwise succeed.

https://cache.nixos.org/log/sk847p5d217wyzvdykkddpd7411k4sp7-librist-0.2.11.drv

meson.build:43:5: ERROR: Program or command 'brew' not found or not executable

this patch will allow builds to work on systems without brew, eg: nixpkgs

diff --git a/meson.build b/meson.build
index 05d00b3..b923958 100755
--- a/meson.build
+++ b/meson.build
@@ -39,7 +39,8 @@ deps = []
 platform_files = []
 inc = []
 inc += include_directories('.', 'src', 'include/librist', 'include', 'contrib')
-if (host_machine.system() == 'darwin')
+if (host_machine.system() == 'darwin'
+    and find_program('brew', required : false).found())
 	r = run_command('brew', '--prefix', check: true)
 	brewoutput = r.stdout().strip()
 	inc += include_directories(brewoutput + '/include')