[cmake] Fix LAPACK/SuiteSparse never being built on Linux

[predat]

Description

When building the dependencies on Linux, LAPACK/BLAS and SuiteSparse were never built, so Ceres ended up configured with CERES_NO_SUITESPARSE and CERES_NO_LAPACK and the sparse bundle adjustment silently fell back to EIGEN_SPARSE.

Root cause: av_conditional_option(... IF "NOT APPLE" ...) receives the quoted condition as a single list element, so if(${_co_IF}) evaluates an undefined variable literally named NOT APPLE and always takes the ELSE branch. This affects AV_BUILD_LAPACK, AV_BUILD_SUITESPARSE and ALICEVISION_REQUIRE_CERES_WITH_SUITESPARSE. That last one is why the AliceVision build did not fail when Ceres had no SuiteSparse support.

Once these dependencies are actually built, two more issues show up. This PR fixes them too.

Features list

• av_conditional_option now evaluates a condition given as a single quoted string (e.g. IF "NOT APPLE") by splitting it into separate arguments. Multi-element conditions are left untouched, so quoted values inside them (e.g. IF X STREQUAL "a b") keep their spaces. The call sites are unchanged.
• LAPACK_CMAKE_FLAGS is now a proper list. It used to be a single string, so Ceres/AliceVision received BLAS_LIBRARIES="<libblas> -DLAPACK_LIBRARIES=<liblapack>". It is joined with spaces only for the CMAKE_OPTIONS variable of SuiteSparse's Makefile.
• Build SuiteSparse with SUITESPARSE_USE_CUDA=OFF. SuiteSparse 7.x enables CUDA by default for all architectures, which greatly increases build time and size. The ENABLE_CUDA option passed so far does not exist in 7.x. The CUDA code is never reached anyway: CHOLMOD only offloads to the GPU through its 64-bit interface (cholmod_l_*), while Ceres, the only SuiteSparse consumer here, uses the 32-bit one.
• Pass BLA_VENDOR=Generic to SuiteSparse along with the reference BLAS/LAPACK paths. Since SuiteSparse 7.x (7.14 in [deps] Update dependency versions and reduce CUDA architectures #2199), giving BLAS_LIBRARIES also requires BLA_VENDOR; without it, SuiteSparse__blas_threading.cmake fails on an empty ${BLA_VENDOR}. SuiteSparse 7.3.0, the version on develop, does not need it but accepts it. When no BLAS/LAPACK paths are given, SuiteSparse still detects the system BLAS itself.
• Download GMP from ftp.gnu.org instead of gmplib.org, which throttles or rejects automated downloads. It is the same archive (same MD5), and MPFR is already downloaded from there.

Implementation remarks

• Checked with CMake 4.2.3:
  • av_conditional_option on Linux: IF "NOT APPLE" and IF "VAR AND NOT APPLE" now give ON (both gave OFF before); IF NOT APPLE, IF VAR, IF APPLE and IF X STREQUAL "a b" give the same result as before.
  • All existing av_conditional_option calls now evaluate as expected: AV_BUILD_LAPACK, AV_BUILD_SUITESPARSE and ALICEVISION_REQUIRE_CERES_WITH_SUITESPARSE are ON, AV_BUILD_OPENMP stays OFF.
  • Through ExternalProject, Ceres receives BLAS_LIBRARIES and LAPACK_LIBRARIES as two separate variables.
  • SuiteSparse's make receives a single CMAKE_OPTIONS=... argument.
• The full dependency build including LAPACK, GMP, MPFR and SuiteSparse has not run yet with these changes. It is the first time this part of the dependency build actually runs on Linux.
• To verify after building the dependencies, CERES_NO_SUITESPARSE and CERES_NO_LAPACK must no longer be defined in include/ceres/internal/config.h.

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