v0.8.9: build fixes and some improvements

Bugfixes and Improvements:

• #274: adding initialization macros for ivec types ( thanks to @duarm )
• #272: revert "fix: simplified pkgconfig generation" ( thanks to @Artturin, also @eli-schwartz, @jtojnar, @q234rty, @bobvanderlinden )
• #273: make implicit double promotion/casting explicit (fixes #228) ( thanks to @JulianSchoenbaechler )
• #276: prevent overriding CMAKE_BUILD_TYPE if it is subdirectory
• see previous releases for more

Sponsorship: Consider become a sponsor for cglm

Supporting cross platform, multiple OS-es and multiple CPUs requires multiple hardwares to test on it which requires extra financial cost. Also sparing time is another big issue while working at somewhere fulltime... Your company can become a sponsor to help:

https://github.com/sponsors/recp
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