External Meson is Required for building on linux

[StormLord07]

Description

When building CPR version 1.12.0 on Linux (Ubuntu Server 22.04) and Windows, CMake errors out with the following message:

CMake Error at build/_deps/cpr-src/cmake/libpsl.cmake:7 (message):
  meson not found.  Please make sure you have meson installed on your system
  (https://mesonbuild.com/Getting-meson.html).  Meson is required for
  building libpsl for curl on Windows.
Call Stack (most recent call first):
  build/_deps/cpr-src/CMakeLists.txt:301 (include)

The Readme doesn't say that any external lib/app is required pre installing the cpr

Example/How to Reproduce

1. git clone https://github.com/libcpr/example-cmake-fetch-content.git
2. cd example-cmake-fetch-content
3. Edit CMakeLists.txt and set CPR version to 1.12.0.
4. cmake . -B build
5. Observe the build fail with Meson-related error.

Possible Fix

Either:

Add meson as an explicitly documented dependency.

Or ensure libpsl is not built or required on Linux unless explicitly enabled.

Where did you get it from?

GitHub (branch e.g. master)

Additional Context/Your Environment

• OS: Linux, Windows
• Version: Ubuntu 24.04.2 LTS, Windows 10

cmake version 3.27.0
c++ (Ubuntu 13.3.0-6ubuntu2~24.04) 13.3.0

[COM8]

@StormLord07 thanks for reporting. There is a CMake option to switch to the system libpsl: CPR_USE_SYSTEM_LIB_PSL

More details: https://github.com/libcpr/cpr/blob/master/CMakeLists.txt#L59C12-L59C34

Inside this CMake option I documented that meson is required.
What would be your preferred way to document the requirement (README.md)?

[StormLord07]

I think the best way'd be in a cmake paragraph

Note:

Since CPR 1.12.0, building the bundled libpsl (used by curl) requires [Meson].
To avoid this, either:

• Install Meson before building, or
• Use the system libpsl by adding -DCPR_USE_SYSTEM_LIB_PSL=ON to your CMake command (requires libpsl to be installed on your system).

or make this a default on option, and introduce a dependancy tab, where we say something like

Dependencies

• libpsl — required by default. CPR will use the system libpsl unless you explicitly disable it.
• To disable and build libpsl from source, set -DCPR_USE_SYSTEM_LIB_PSL=OFF (requires [Meson] to be installed).

Just mension libps and mason more so people could find the solution faster

[COM8]

I'm working on this here: #1253