Mac OS: Fix gettext lookup in deps build

deps/deps-macos.cmake

@@ -111,6 +111,6 @@ ExternalProject_Add(dep_wxwidgets
         --with-expat=builtin
         --disable-debug
         --disable-debug_flag
-    BUILD_COMMAND make "-j${NPROC}" && make -C locale allmo
+    BUILD_COMMAND make "-j${NPROC}" && PATH=/usr/local/opt/gettext/bin/:$ENV{PATH} make -C locale allmo
     INSTALL_COMMAND make install
 )

doc/How to build - Mac OS.md

@@ -1,7 +1,15 @@
 
 # Building PrusaSlicer on Mac OS
 
-To build PrusaSlicer on Mac OS, you will need to install XCode, [CMake](https://cmake.org/) (available on Brew) and possibly git.
+To build PrusaSlicer on Mac OS, you will need the following software:
+
+- XCode
+- CMake
+- git
+- gettext
+
+XCode is available through Apple's App Store, the other three tools are available on
+[brew](https://brew.sh/) (use `brew install cmake git gettext` to install them).
 
 ### Dependencies