[build] Set LLVM_ENABLE_ZLIB=FORCE_ON in LLVM build system

This makes tests to check zlib is usable error out loudly, instead of silently
proceeding with a broken build.

Author: giordano

deps/llvm.mk

@@ -94,7 +94,7 @@ LLVM_LDFLAGS += $(BOLT_LDFLAGS)
 LLVM_CMAKE += -DLLVM_TARGETS_TO_BUILD:STRING="$(LLVM_TARGETS)" -DCMAKE_BUILD_TYPE="$(LLVM_CMAKE_BUILDTYPE)"
 LLVM_CMAKE += -DLLVM_EXPERIMENTAL_TARGETS_TO_BUILD:STRING="$(LLVM_EXPERIMENTAL_TARGETS)"
 LLVM_CMAKE += -DLLVM_ENABLE_LIBXML2=OFF -DLLVM_HOST_TRIPLE="$(or $(XC_HOST),$(BUILD_MACHINE))"
-LLVM_CMAKE += -DLLVM_ENABLE_ZLIB=ON -DZLIB_ROOT="$(build_prefix)"
+LLVM_CMAKE += -DLLVM_ENABLE_ZLIB=FORCE_ON -DZLIB_ROOT="$(build_prefix)"
 LLVM_CMAKE += -DLLVM_ENABLE_ZSTD=OFF
 ifeq ($(USE_POLLY_ACC),1)
 LLVM_CMAKE += -DPOLLY_ENABLE_GPGPU_CODEGEN=ON