Find OpenMP package with REQUIRED keyword (!93)

## What is the bug
A package-NOTFOUND is undetected and does not raise a problem. See #20
This MR closes #20
## How do you fix it
The OpenMP find package is made `REQUIRED`.  
The find package for OpenACC has to stay `QUIET` or else it will try to look for both `OpenACC_C` and `OpenACC_Fortran`. (LUMI does not have `OpenACC_C` at the moment.)

Approved-by: Jonas Jucker <jonas.jucker@env.ethz.ch>
Approved-by: Sergey Kosukhin <sergey.kosukhin@mpimet.mpg.de>
Merged-by: Yen-Chen Chen <yen-chen.chen@kit.edu>
Changelog: other

src/CMakeLists.txt

@@ -19,7 +19,7 @@ configure_file(${CMAKE_CURRENT_SOURCE_DIR}/config.h.in
                ${CMAKE_CURRENT_BINARY_DIR}/config.h)
 
 if(FS_ENABLE_OMP)
-  find_package(OpenMP QUIET)
+  find_package(OpenMP REQUIRED)
   set(CMAKE_Fortran_FLAGS "${CMAKE_Fortran_FLAGS} ${OpenMP_Fortran_FLAGS}")
 endif()