Add more customizations to Crystal::Config.description

* Allow passing in git sha and date to Makefile
* Remove empty parens when date is ""
* Add message when compiler was not compiled in release mode

When building in a nix derivation, there is no git directory, so you
have to pass in the sha manually. Also, all the file modification
times are set to epcoh, and I think it'd be nicer to just have the
builds not have a date rather than the obviously wrong 1970-01-01.

I don't think we need to always have release mode information, but it
would be useful I think to have a way to see when you're working with a
non-release compiler.

Makefile

@@ -39,9 +39,9 @@ override FLAGS += -D strict_multi_assign -D preview_overload_order $(if $(releas
 SPEC_WARNINGS_OFF := --exclude-warnings spec/std --exclude-warnings spec/compiler --exclude-warnings spec/primitives
 SPEC_FLAGS := $(if $(verbose),-v )$(if $(junit_output),--junit_output $(junit_output) )$(if $(order),--order=$(order) )
 CRYSTAL_CONFIG_LIBRARY_PATH := '$$ORIGIN/../lib/crystal'
-CRYSTAL_CONFIG_BUILD_COMMIT := $(shell git rev-parse --short HEAD 2> /dev/null)
+CRYSTAL_CONFIG_BUILD_COMMIT ?= $(shell git rev-parse --short HEAD 2> /dev/null)
 CRYSTAL_CONFIG_PATH := '$$ORIGIN/../share/crystal/src'
-SOURCE_DATE_EPOCH := $(shell (git show -s --format=%ct HEAD || stat -c "%Y" Makefile || stat -f "%m" Makefile) 2> /dev/null)
+SOURCE_DATE_EPOCH ?= $(shell (git show -s --format=%ct HEAD || stat -c "%Y" Makefile || stat -f "%m" Makefile) 2> /dev/null)
 ifeq ($(shell command -v ld.lld >/dev/null && uname -s),Linux)
   EXPORT_CC ?= CC="$(CC) -fuse-ld=lld"
 endif

src/compiler/crystal/config.cr

@@ -16,11 +16,14 @@ module Crystal
 
     def self.description
       formatted_sha = "[#{build_commit}] " if build_commit
+      formatted_date = "(#{date})" unless date.empty?
+      release_msg = "\nRelease mode: false" unless release_mode
+
       <<-DOC
-        Crystal #{version} #{formatted_sha}(#{date})
+        Crystal #{version} #{formatted_sha}#{formatted_date}
 
         LLVM: #{llvm_version}
-        Default target: #{self.host_target}
+        Default target: #{self.host_target}#{release_msg}
         DOC
     end
 
@@ -40,6 +43,14 @@ module Crystal
       end
     end
 
+    def self.release_mode
+      {% if flag?(:release) %}
+        true
+      {% else %}
+        false
+      {% end %}
+    end
+
     @@host_target : Crystal::Codegen::Target?
 
     def self.host_target : Crystal::Codegen::Target