Fix GNUInstallDirs includedir detection

[viccie30]

CMake uses INCLUDEDIR instead of INCDIR.

[codecov]

Codecov Report

Merging #891 (b1459a4) into develop (03051a0) will decrease coverage by 0.59%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #891      +/-   ##
===========================================
- Coverage    75.43%   74.83%   -0.60%     
===========================================
  Files           73       73              
  Lines         8220     7782     -438     
  Branches      1359     1359              
===========================================
- Hits          6201     5824     -377     
+ Misses        1494     1420      -74     
- Partials       525      538      +13     

Flag	Coverage Δ
macos_clang	68.73% <ø> (ø)
macos_gcc	67.76% <ø> (ø)
ubuntu_clang	68.92% <ø> (ø)
ubuntu_clang_debug	68.36% <ø> (ø)
ubuntu_clang_inflate_allow_invalid_dist	68.91% <ø> (ø)
ubuntu_clang_inflate_strict	69.17% <ø> (ø)
ubuntu_clang_mmap	68.92% <ø> (ø)
ubuntu_clang_msan	69.17% <ø> (ø)
ubuntu_gcc	67.06% <ø> (-2.79%)	⬇️
ubuntu_gcc_aarch64	70.07% <ø> (ø)
ubuntu_gcc_aarch64_compat_no_opt	67.78% <ø> (ø)
ubuntu_gcc_aarch64_no_acle	68.30% <ø> (ø)
ubuntu_gcc_aarch64_no_neon	67.99% <ø> (ø)
ubuntu_gcc_armhf	70.49% <ø> (ø)
ubuntu_gcc_armhf_compat_no_opt	67.77% <ø> (ø)
ubuntu_gcc_armhf_no_acle	69.54% <ø> (ø)
ubuntu_gcc_armhf_no_neon	69.86% <ø> (ø)
ubuntu_gcc_armsf	70.48% <ø> (ø)
ubuntu_gcc_armsf_compat_no_opt	67.77% <ø> (ø)
ubuntu_gcc_compat_no_opt	69.14% <ø> (ø)
ubuntu_gcc_mingw_i686	0.00% <ø> (ø)
ubuntu_gcc_mingw_x86_64	0.00% <ø> (ø)
ubuntu_gcc_no_avx2	69.41% <ø> (ø)
ubuntu_gcc_no_pclmulqdq	66.52% <ø> (ø)
ubuntu_gcc_no_sse2	67.77% <ø> (ø)
ubuntu_gcc_no_sse4	66.52% <ø> (ø)
ubuntu_gcc_o3	69.12% <ø> (ø)
ubuntu_gcc_osb	?
ubuntu_gcc_ppc	70.91% <ø> (ø)
ubuntu_gcc_ppc64	70.08% <ø> (ø)
ubuntu_gcc_ppc64le	69.06% <ø> (ø)
ubuntu_gcc_s390x	68.27% <ø> (ø)
ubuntu_gcc_sparc64	70.79% <ø> (ø)
win64_gcc	73.02% <ø> (ø)
win64_gcc_compat_no_opt	74.46% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
inftrees.c	76.92% <0.00%> (-4.38%)	⬇️
gzwrite.c	48.75% <0.00%> (-4.19%)	⬇️
test/adler32_test.c	60.00% <0.00%> (-3.64%)	⬇️
test/fuzz/minigzip_fuzzer.c	58.33% <0.00%> (-2.66%)	⬇️
gzlib.c	54.47% <0.00%> (-2.52%)	⬇️
test/fuzz/checksum_fuzzer.c	92.00% <0.00%> (-2.24%)	⬇️
test/minideflate.c	57.30% <0.00%> (-2.14%)	⬇️
gzread.c	54.60% <0.00%> (-1.90%)	⬇️
inffast.c	86.52% <0.00%> (-1.75%)	⬇️
crc32_comb.c	80.48% <0.00%> (-1.74%)	⬇️
... and 40 more

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[Dead2]

Oh, I missed those two. 🤦‍♂️