cmake: Fix check_arm32_assembly when using as subproject #1526

1. 
   hebasto commented at 11:40 am on May 7, 2024: member

   When integrating libsecpk1 in a downstream project like this:

   0set(SECP256K1_ASM arm32 CACHE STRING "" FORCE)
   1add_subdirectory(src/secp256k1)
   

   it fails to configure:

    0CMake Error at /home/hebasto/git/bitcoin/build/check_arm32_assembly/CMakeFiles/CMakeTmp/CMakeLists.txt:21 (target_sources):
    1  Cannot find source file:
    2
    3    /home/hebasto/git/bitcoin/cmake/source_arm32.s
    4
    5
    6CMake Error at /home/hebasto/git/bitcoin/build/check_arm32_assembly/CMakeFiles/CMakeTmp/CMakeLists.txt:20 (add_executable):
    7  No SOURCES given to target: cmTC_d0f0b
    8
    9
   10CMake Error at src/secp256k1/cmake/CheckArm32Assembly.cmake:2 (try_compile):
   11  Failed to generate test project build system.
   12Call Stack (most recent call first):
   13  src/secp256k1/CMakeLists.txt:127 (check_arm32_assembly)
   

   This PR fixes this issue, which was overlooked in #1304.

2. cmake: Fix `check_arm32_assembly` when using as subproject 9f4c8cd730
3. 
   hebasto commented at 11:41 am on May 7, 2024: member

   cc @real-or-random @theuni
4. real-or-random added the label build on May 7, 2024
5. real-or-random added the label bug on May 7, 2024
6. real-or-random approved
7. 
   real-or-random commented at 11:44 am on May 7, 2024: contributor

   utACK 9f4c8cd730bba94f0c09cbef9b0cf9df44eb7b03

   Yes, this makes sense according to the docs.

8. theuni approved
9. 
   theuni commented at 5:14 pm on May 8, 2024: contributor

   utACK 9f4c8cd730bba94f0c09cbef9b0cf9df44eb7b03
10. real-or-random merged this on May 9, 2024
11. real-or-random closed this on May 9, 2024

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12. hebasto deleted the branch on May 9, 2024

Contributors
hebasto real-or-random theuni

Labels
bug build