Add CMAKE_C_FLAGS, remove COMMAND_ECHO #2491

Job	Run time
downstream	45m 31s
linux-compat (manylinux1-x64)	1m 13s
linux-compiler-compat (clang-3)	1m 47s
linux-compat (manylinux1-x86)	1m 15s
linux-compiler-compat (clang-6)	6m 2s
linux-compat (manylinux2014-x64)	1m 19s
linux-compiler-compat (clang-8)	6m 34s
linux-compat (manylinux2014-x86)	1m 18s
linux-compiler-compat (clang-9)	6m 5s
linux-compat (raspbian-bullseye)	7m 0s
clang-sanitizers (,thread)	2m 22s
linux-compiler-compat (clang-10)	2m 0s
linux-compat (fedora-34-x64)	1m 18s
linux-compiler-compat (clang-11)	1m 55s
clang-sanitizers (,address,undefined)	1m 50s
linux-compat (opensuse-leap)	1m 36s
linux-compiler-compat (gcc-4.8)	1m 34s
linux-no-cpu-extensions	1m 34s
linux-compat (rhel8-x64)	1m 6s
linux-compiler-compat (gcc-5)	1m 47s
linux-compat (al2-x64)	1m 13s
linux-compiler-compat (gcc-6)	1m 34s
linux-shared-libs	1m 34s
windows	1m 58s
linux-compiler-compat (gcc-7)	1m 49s
windows-vc16 (x86)	2m 31s
windows-app-verifier	27m 5s
windows-vc16 (x64)	2m 31s
linux-compiler-compat (gcc-8)	2m 1s
linux-compiler-compat (gcc-11)	2m 16s
windows-shared-libs	2m 58s
windows-no-cpu-extensions	2m 51s
windows-vc14 (x86)	1m 52s
windows-vc14 (x64)	1m 31s
macos	1m 19s
windows-vc15 (x86)	2m 48s
macos-x64	2m 39s
windows-vc15 (x64)	2m 58s
freebsd	9m 13s
macos-no-cpu-extensions	1m 21s
openbsd	12m 2s
Cross Compile linux-armv6	1m 58s
Cross Compile linux-armv7	1m 56s
linux-debug	1m 28s
Cross Compile linux-arm64	1m 39s
Cross Compile android-armv7	2m 5s
windows-debug	2m 19s
macos-debug	1m 21s
	3h 13m 56s

Provide feedback