Update C++17 requirement cases

C++17 is required to build UMF only when two flags are enabled:
- UMF_BUILD_LIBUMF_POOL_DISJOINT,
- UMF_BUILD_TESTS.
In other cases, all is needed is a C compiler.

Author: PatKamin

README.md

@@ -72,12 +72,15 @@ The `UMF_BUILD_LIBUMF_POOL_SCALABLE` option has to be turned `ON` to build this
 ### Requirements
 
 Required packages:
-- C++ compiler with C++17 support
+- C compiler
 - [CMake](https://cmake.org/) >= 3.14.0
 
 For development and contributions:
 - clang-format-15.0 (can be installed with `python -m pip install clang-format==15.0.7`)
 
+For building tests and Disjoint Pool:
+- C++ compiler with C++17 support
+
 ### Benchmark
 
 A simple micro benchmark based on [ubench](https://github.com/sheredom/ubench.h).