configure: explicitly disable spirv_compiler

spirv_compiler is an undeclared variable that was spontaneously
enabled if libshaderc or libglslang were detected, and served as
a way to enable filters.

However, it being undeclared had the effect that it was neither
considered explicitly disabled nor enabled if libshaderc or
libglslang were detected.

The only category we have which explicitly disables variables
is EXTERNAL_LIBRARY_LIST, however, spirv_compiler is not a real
external library, and making it present there would report its
detection on ./configure.

configure

@@ -6553,6 +6553,15 @@ if test -n "$custom_allocator"; then
     add_extralibs "$custom_allocator_extralibs"
 fi
 
+# Unlike other feature flags or libraries, spirv_compiler is not defined
+# within any of our predefined categories of components.
+# It gets defined if either libshaderc or libglslang check succeeds.
+# As such, its in a state of neither being explicitly enabled, nor
+# explicitly disabled, but even in this state, being mentioned in
+# _deps results in it always passing.
+# Disable it explicitly to fix this.
+disable spirv_compiler
+
 check_func_headers malloc.h _aligned_malloc     && enable aligned_malloc
 check_func  ${malloc_prefix}memalign            && enable memalign
 check_func  ${malloc_prefix}posix_memalign      && enable posix_memalign