Change default -ccompilers to "clang,gcc"

Updates #62

cmd/commonflags/commonflags.go

diff --git a/cmd/commonflags/commonflags.go b/cmd/commonflags/commonflags.go
index 10cd4f8..c9dec92 100644
--- a/cmd/commonflags/commonflags.go
+++ b/cmd/commonflags/commonflags.go
@@ -27,7 +27,7 @@
 )
 
 const (
-	CcompilersDefault = "clang-9,gcc"
+	CcompilersDefault = "clang,gcc"
 	CcompilersUsage   = `comma-separated list of C compilers`
 
 	FocusDefault = ""

doc/changelog.md

diff --git a/doc/changelog.md b/doc/changelog.md
index ac3db69..56747a3 100644
--- a/doc/changelog.md
+++ b/doc/changelog.md
@@ -40,7 +40,7 @@
 - Added slice `uintptr_low_12_bits` method.
 - Added tokens.
 - Changed `gif.decoder_workbuf_len_max_incl_worst_case` from 1 to 0.
-- Changed default C compilers from `clang-5.0,gcc` to `clang-9,gcc`.
+- Changed default C compilers from `clang-5.0,gcc` to `clang,gcc`.
 - Changed the C formatting style; removed the `-cformatter` flag.
 - Changed what the `std/gif` benchmarks actually measure.
 - Made `wuffs_base__pixel_format` a struct.