CMakeLists.txt: fix build with ninja

[ThomasDevoogdt]

$ cmake -GNinja -B build/ && cmake --build build/

Results in this error:

ninja: error: build.ninja:158: bad $-escape (literal $ must be written as $$)

Replacing the $(MAKE) command with make gives us this new error:

ninja: error: 'backtrace-prefix/lib/libbacktrace.a', needed by 'bin/fluent-bit', missing and no known rule to make it

So fix that by properly defining the BUILD_BYPRODUCTS. (Also see https://cmake.org/cmake/help/latest/module/ExternalProject.html#build-step-options)

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Testing
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Backporting

• Backport to latest stable release.

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Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.

[ThomasDevoogdt]

@edsiper, can you consider this PR?

[cosmo0920]

Just specifying fixed "make" is not proper option for Windows.
Instead, how about using the following lines:

if (CMAKE_GENERATOR MATCHES "Ninja")
  if (FLB_SYSTEM_WINDOWS)
    MESSAGE(FATAL_ERROR "Building with Ninja is not supported on Windows")
  else()
    set(EXTERNAL_BUILD_TOOL "make")
  endif()
else()
  set(EXTERNAL_BUILD_TOOL "$(MAKE)")
endif()

and using ${EXTERNAL_BUILD_TOOL} in the following ExternalProject_Add?
This could be supporting Windows environment and preserve the previous behavior without specifying Ninja genator.