Update build/install instructions for custom install dir translations to work (flameshot-org#3821)

Author: MatthiasKunnen

README.md

@@ -401,7 +401,7 @@ Also you can open and build/debug the project in a C++ IDE. For example, in Qt C
 - Qt >= 5.9
   + Development tools
 - GCC >= 7.4
-- CMake >= 3.13
+- CMake >= 3.29
 
 #### Run-time
 
@@ -461,50 +461,64 @@ nix-shell
 
 #### macOS
 
-First of all you need to install [brew](https://brew.sh) and than install the dependencies
+First of all you need to install [brew](https://brew.sh) and then install the dependencies
 ```shell
 brew install qt5
 brew install cmake
 ```
 
 ### Build
 
-After installing all the dependencies, finally run the following commands in the sources root directory:
+After installing all the dependencies, flameshot can be built.
 
-```shell
-cmake -S . -B build && cmake --build build
-```
-
-NOTE: For macOS you should replace the command
+#### Installation/build dir
+For the translations to be loaded correctly, the build process needs to be aware of where you want
+to install flameshot.
 
 ```shell
-cmake -S . -B build
+# Directory where build files will be placed, may be relative
+export BUILD_DIR=build
+
+# Directory prefix where flameshot will be installed. If you are just building and don't want to
+# install, comment this environment variable.
+# This excludes the bin/flameshot part of the install,
+# e.g. in /opt/flameshot/bin/flameshot, the CMAKE_INSTALL_PREFIX is /opt/flameshot
+# This must be an absolute path. Requires CMAKE 3.29.
+export CMAKE_INSTALL_PREFIX=/opt/flameshot
+
+# Linux
+cmake -S . -B "$BUILD_DIR" \
+    && cmake --build "$BUILD_DIR"
+
+#MacOS
+cmake -S . -B "$BUILD_DIR" \
+    -DQt5_DIR="$(brew --prefix qt5)/lib/cmake/Qt5" \
+    && cmake --build "$BUILD_DIR"
 ```
 
-with
-
-```shell
-cmake -S . -B build -DQt5_DIR=$(brew --prefix qt5)/lib/cmake/Qt5
-```
-
-When the `cmake --build build` command has completed you can launch flameshot from the `project_folder/build/src` folder.
+When the `cmake --build` command has completed you can launch flameshot from the `project_folder/build/src` folder.
 
 ### Install
 
 Note that if you install from source, there _is no_ uninstaller, so consider installing to a custom directory.
 
 #### To install into a custom directory
+Make sure you are using cmake `>= 3.29` and build flameshot with `$CMAKE_INSTALL_PREFIX` set to the
+installation directory. If this is not done, the translations won't be found when using a custom directory.
+Then, run the following:
+
 ```bash
-# Best to use an absolute path here
-INST_DIR=/opt/flameshot
+# !Build with CMAKE_INSTALL_PREFIX and use cmake >= 3.29! Using an older cmake will cause
+# installation into the default /usr/local dir.
+
 # You may need to run this with privileges
-cmake --install build --prefix "$INST_DIR"
+cmake --install "$BUILD_DIR"
 ```
 
 #### To install to the default install directory
 ```bash
 # You may need to run this with privileges
-cmake --install build
+cmake --install "$BUILD_DIR"
 ```
 
 ### FAQ