Fix CMake compilation with -Wp,-D_FORTIFY_SOURCE=2 flags

[aale24]

Handle both -D_FORTIFY_SOURCE=2 and -Wp,-D_FORTIFY_SOURCE=2 variants when removing FORTIFY_SOURCE flags for unit test compilation with -O0.

What does this PR do?

Fixes a CMake compilation error that occurs when building GDAL unit tests on systems using hardened compiler flags with the -Wp,-D_FORTIFY_SOURCE=2 format.

The existing code in autotest/cpp/CMakeLists.txt only removes the -D_FORTIFY_SOURCE=2 variant when compiling unit tests with -O0 optimization, but doesn't handle the -Wp,-D_FORTIFY_SOURCE=2 variant used by modern hardened build environments. This leaves dangling -Wp, prefixes that cause compilation to fail with "too many filenames" errors.

The fix adds string replacement for both variants to ensure clean flag removal.

What are related issues/pull requests?

Fixes #12840

Tasklist

• AI (Copilot or something similar) supported my development of this PR
• Make sure code is correctly formatted (cf pre-commit configuration)
• Add test case(s)
• Add documentation
• Updated Python API documentation (swig/include/python/docs/)
• Review
• Adjust for comments
• All CI builds and checks have passed

Environment

Provide environment details, if relevant:

• OS: Amazon Linux 2023
• Compiler: GCC with hardened flags (-Wp,-D_FORTIFY_SOURCE=2)

Testing

cmake .. -DCMAKE_CXX_FLAGS="-Wp,-D_FORTIFY_SOURCE=2 -O2"
cmake --build . --target gdal_unit_test --verbose

gdal-fixed-with-Wp.log

cmake .. -DCMAKE_CXX_FLAGS="-D_FORTIFY_SOURCE=2 -O2"
-D_FORTIFY_SOURCE=2 -O2"
cmake --build . --target gdal_unit_test --verbose

gdal-fixed-without-Wp.log

cmake .. 
-D_FORTIFY_SOURCE=2 -O2"
cmake --build . --target gdal_unit_test --verbose

gdal-fixed-no-flags.log