PKG_CHECK_MODULES in pkg.m4 unconditionally declares precious variables AC_ARG_VAR([$1][_CFLAGS], [C compiler flags for $1, overriding pkg-config])dnl AC_ARG_VAR([$1][_LIBS], [linker flags for $1, overriding pkg-config])dnl which means several more pages of text printed by ./configure --help that include every argument to PKG_CHECK_MODULES. For instance for my app it now prints internal dummy flags that are used in an auxiliary check and never included actual in CFLAGS or LDFLAGS: FFTW3_DUMMY_CFLAGS C compiler flags for FFTW3_DUMMY, overriding pkg-config FFTW3_DUMMY_LIBS linker flags for FFTW3_DUMMY, overriding pkg-config This is confusing. The user should never see this. configure.ac author should have a possibility to limit the output to meaningful variables.
You should use PKG_CHECK_EXISTS if you don't want to set variables; that macro does not have side effects apart from what you specify yourself in the second and third argument.
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