Use @static if for compile-time availability checks

Changed the availability check functions to use @static if for compile-time
determination, matching the pattern used in AppleAccelerate:
- Uses @static if to check @isdefined at compile time
- Returns is_available() result statically when JLL is defined
- Returns false statically when JLL is not defined

This makes the checks more efficient as they're resolved at compile time
rather than runtime.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: claude

src/mkl.jl

@@ -9,11 +9,10 @@ to avoid allocations and does not require libblastrampoline.
 struct MKLLUFactorization <: AbstractFactorization end
 
 # Check if MKL is available
-function __mkl_isavailable()
-    if !@isdefined(MKL_jll)
-        return false
-    end
-    return MKL_jll.is_available()
+@static if !@isdefined(MKL_jll)
+    __mkl_isavailable() = false
+else
+    __mkl_isavailable() = MKL_jll.is_available()
 end
 
 function getrf!(A::AbstractMatrix{<:ComplexF64};

src/openblas.jl

@@ -34,11 +34,10 @@ sol = solve(prob, OpenBLASLUFactorization())
 struct OpenBLASLUFactorization <: AbstractFactorization end
 
 # Check if OpenBLAS is available
-function __openblas_isavailable()
-    if !@isdefined(OpenBLAS_jll)
-        return false
-    end
-    return OpenBLAS_jll.is_available()
+@static if !@isdefined(OpenBLAS_jll)
+    __openblas_isavailable() = false
+else
+    __openblas_isavailable() = OpenBLAS_jll.is_available()
 end
 
 function openblas_getrf!(A::AbstractMatrix{<:ComplexF64};