Properly export ffs in adios2-config when SST is disabled.

[vicentebolea]

The issue happens because EVPATH dependency gets properly exported when using SST, however, even without SST we use ffs which gets exposed through EVPATH. @ax3l the workaround should be to enable SST or disable BP5.

Things to do:

• Properly export ffs in adios2-config when SST is disabled.

Originally posted by @vicentebolea in #3716 (comment)

[ax3l]

Note: this issue is only with static builds:

cmake -S . -B build \
  -DADIOS2_USE_SST=OFF \
  -DBUILD_SHARED_LIBS=OFF

it appears when a dependent CMake project tries to include ADIOS2 CMake targets from adios2-config.cmake.

Originally seen on macOS 11.0 for cross-compiles from x86 to arm64/aarch64 with the 2.9.0 release.

[vicentebolea]

@scottwittenburg

[vicentebolea]

Solved in #3760