main/buho: fix build

It seems the the CFLAGS are not properly set in the CMakeLists.txt.
Cmake currently appends -std=gnu90 at the end of the CFLAGS, overriding
the earlier set -std=c99. Work around this with sed. Better fix is
welcome, this is a stop-gap solution to unbreak the build (which is
blocking build.postmarketos.org).

main/buho/APKBUILD

@@ -2,7 +2,7 @@
 # Maintainer: Bart Ribbers <bribbers@disroot.org>
 pkgname=buho
 pkgver=0_git20190924
-pkgrel=0
+pkgrel=1
 _commit="375480f43ca3f1eefb0fe029582426ad94e760ae"
 pkgdesc="Note taking app based on MauiKit"
 arch="all"
@@ -19,6 +19,10 @@ build() {
 		-DCMAKE_BUILD_TYPE=RelWithDebInfo \
 		-DCMAKE_INSTALL_PREFIX=/usr \
 		-DCMAKE_INSTALL_LIBDIR=lib
+
+	# Workaround for broken CMakeLists.txt
+	sed -i s/std=gnu90/std=c99/g CMakeFiles/buho.dir/flags.make
+
 	make
 }